Charles Dillon Stengel was born on July 30, 1890, in Kansas City, Missouri. His ancestry involves German and Irish; his parents—Louis Stengel and Jennie (Wolff) Stengel—were from the Quad Cities area of Illinois and Iowa, and had moved to Kansas City soon after their 1886 wedding so Louis could take an insurance job. "Charlie" was the youngest of three children, and the second son. Charlie Stengel played sandlot baseball as a child, and also played baseball, football and basketball at Kansas City's Central High School. His basketball team won the city championship, while the baseball team won the state championship.

As a teenager, Stengel played on a number of semipro baseball teams. He played on the traveling team called the Kansas City Red Sox during the summers of 1908 and 1909, going as far west as Wyoming and earning a dollar a day. He was offered a contract by the minor league Kansas City Blues for $135 a month, more money than his Father was making. Since young Stengel was underage, his Father had to agree, which he did. Louis Stengel recalled, "So I put down my paper and signed. You could never change that boy's mind anyway".

Kansas City optioned Stengel to the Kankakee Kays of the Class D Northern Association, a lower-level minor league, to gain experience as an outfielder. He had a .251 batting average with Kankakee when the league folded in July. He found a place with the Shelbyville Grays, who moved mid-season and became the Maysville Rivermen, of the Class D Blue Grass League, batting .221. He returned to the Blues for the final week of the season, with his combined batting average for 1910 at .237.

Uncertain of whether he would be successful as a baseball player, Stengel attended Western Dental College in the 1910–1911 offseason. He would later tell stories of his woes as a left-handed would-be dentist using right-handed equipment. The Blues sold Stengel to the Aurora Blues of the Class C Wisconsin–Illinois League. He led the league with a .352 batting average. Brooklyn Dodgers scout Larry Sutton took a trip from Chicago to nearby Aurora, noticed Stengel, and the Dodgers purchased his contract on September 1, 1911. Brooklyn outfielder Zach Wheat later claimed credit for tipping off Sutton that Stengel was worth signing. Stengel finished the season with Aurora and returned to dental school for the offseason.

In later years, Casey Stengel told stories of his coming to Brooklyn to play for the Dodgers; most focused on his naïveté and were, at least, exaggerated. Wheat was from the Kansas City area and watched over Stengel, getting the young player a locker next to his and working with him on outfield technique. Stengel made his MLB debut at Brooklyn's Washington Park on September 17, 1912, as the starting center fielder, and went 4–4 with a walk. He played well with the Dodgers to finish with a .316 batting average, though hitting .351 when right handers started against Brooklyn and only .250 when left handers started.

After holding out for better pay, Stengel signed with the Dodgers for 1913. He won the starting center fielder job. The Dodgers had a new ballpark, Ebbets Field, and Stengel became the first person to hit a home run there, first an inside-the-park home run against the New York Yankees in an exhibition game to open the stadium, and then in the regular season. During the 1913 season, Stengel acquired the nickname "Casey"; there are varying stories of how this came to be, though his home town of Kansas City likely played a prominent role—sportswriter Fred Lieb stated that the ballplayer had "Charles Stengel—K.C." stenciled on his bags. Although he missed 25 games with injuries, he hit .272 with 7 official home runs in his first full season as a major leaguer.

Stengel re-invigorated platooning in baseball. Most of Stengel's playing days were after offensive platooning, choosing left-handed batters against a right-handed pitcher and the converse, was used successfully by the World Series champion 1914 Boston "Miracle" Braves. This set off widespread use of platooning: Stengel himself was several times during his playing career platooned against right-handers, but the practice fell into disuse around 1930. Stengel reintroduced it to the Yankees, and its prominent use amid the team's success caused it to be imitated by other teams. Bill James deemed Stengel and Earl Weaver the most successful platoon managers.

Stengel reported to spring training ill and thin; he was unable to work out for much of the time the Dodgers spent in Florida. Although the team stated that he had typhoid fever, still Common in 1915, Lieb wrote after Stengel's death in 1975 that the ballplayer had gonorrhea. Stengel may have been involved in a well-known incident during spring training when Robinson agreed to catch a baseball dropped from an airplane piloted by Ruth Law—except that it proved not to be a baseball, but a small grapefruit, much to the manager's shock, as he assumed the liquid on him was blood. Law stated that she dropped the grapefruit as she had forgotten the baseball, but Stengel retold the story, imitating Robinson, many times in his later years, with himself as grapefruit dropper, and is often given the credit for the stunt. Stengel's batting average dipped as low as the .150s for part of the season; though he eventually recovered to .237, this was still the worst full season percentage of his major league career.

Soon after the 1916 season started, it became clear that the Dodgers were one of the better teams in the league. Robinson managed to squeeze one more year of productivity out of some older veterans such as Chief Meyers, who said of Stengel, "It was Casey who kept us on our toes. He was the life of the party and just kept all us old-timers pepped up all season." Stengel, mostly playing right field, hit .279 with eight home runs, one less than the team leader in that category, Wheat. Stengel's home run off Philadelphia Phillies pitcher Grover Cleveland Alexander, who won 33 games that year, in the second game of a doubleheader on September 30, provided the margin of victory as the Dodgers went into first place to stay, qualifying for their first World Series, against the Boston Red Sox. Despite getting two hits in a Game 1 loss at Boston, Stengel was benched for Game 2 because the Red Sox were pitching a left hander, Babe Ruth, and Stengel hit better against right handers. Stengel, back in the lineup for Game 3, got a hit in Brooklyn's only victory of the series. He was benched again in Game 4 against left hander Dutch Leonard, though he was inserted as a pinch Runner, and got another hit in the Game 5 loss, finishing 4 for 11, .364, the best Dodger batting average of the Series.

Despite the successful season, Ebbets was determined to cut his players' salaries, including Stengel, whom he considered overpaid. By then, the Federal League was defunct, and the reserve clause prevented players from jumping to other major league clubs. The owner sent Stengel a contract for $4,600, and when that was rejected, cut it by another $400. A holdout ensured, together with a war of words waged in the press. With little leverage, Stengel became willing to sign for the original contract, and did on March 27, 1917, but missed most of spring training. Stengel's batting average dropped from .279 to .257 as the defending league champion Dodgers finished seventh in the eight-team league, but he led the team in games, hits, doubles, triples, home runs and runs batted in. After the season, Ebbets sent Stengel a contract for $4,100, and the outfielder eventually signed for that amount, but on January 9, 1918, Ebbets traded him along with George Cutshaw to the Pittsburgh Pirates for Burleigh Grimes, Al Mamaux and Chuck Ward.

The Pirates had been the only National League team to do worse than the Dodgers in 1917, finishing last. Stengel met with the Pirates owner Barney Dreyfuss to seek a salary increase, but found Dreyfuss reluctant to deal until Stengel proved himself as a Pirate. On June 3, 1918, Stengel was ejected for arguing with the Umpire, and was fined by the league office for taking off his shirt on the field. The U.S. had been fighting in World War I for a year, and Stengel enlisted in the Navy. His wartime Service was playing for and managing the Brooklyn Navy Yard's baseball team, driving in the only run to beat Army, 1–0, before 5,000 spectators at the Polo Grounds. He also occasionally helped paint a ship—he later stated he had guarded the Gowanus Canal, and not a single submarine got into it.

The Armistice renewed the war between Stengel and Dreyfuss, and the outfielder held out again to begin the 1919 season. Both wanted to see Stengel traded, but no deal was immediately made. By then, Stengel's position as regular right fielder had been taken by Billy Southworth, and he had difficulty breaking back into the lineup. Stengel played better than he had before he enlisted, and by the time Dreyfuss traded him to the Philadelphia Phillies for Possum Whitted on August 9, he was batting .293 with four home runs. But before being traded, Stengel pulled one of his most famous Stunts, on May 25 at Ebbets Field, as a member of the visiting Pirates. It was not unusual at Ebbets Field for right fielders of either team, rather than go to the dugout after three were out, to go to the Dodgers' bullpen, in foul territory down the right field line, if they were not likely to bat in the upcoming inning. Stengel did so to visit old friends, and discovered that pitcher Leon Cadore had captured a sparrow. Stengel took it, and, called to bat in the sixth inning, quietly placed it under his cap. He received mixed boos and cheers from the Brooklyn crowd as a former Dodger, took a deep bow at the plate, and doffed his cap, whereupon the bird flew away to great laughter from the crowd.

When Stengel reported to the Giants on July 1, 1921, they were managed, as they had been for almost 20 years, by John McGraw. Stengel biographer Robert W. Creamer said of McGraw, "his Giants were the most feared, the most respected, the most admired team in baseball". Stengel biographer Marty Appel noted, "McGraw and Stengel. Teacher and student. Casey was about to learn a lot about managing". Stengel had time to learn, playing in only 18 games for the Giants in 1921, mostly as a pinch hitter, and watched from his place on the bench as McGraw led the Giants from a 7​⁄₂ game deficit on August 24 to the National League pennant. Although he was on the 25-player postseason roster, Stengel did not appear in the 1921 World Series against the Yankees, as McGraw used only 13 players (4 of them pitchers) in beating the Yankees, five games to three. The only contribution Stengel made to the box score was being ejected from Game 5 for arguing.

McGraw brought several outfielders into spring training with the Giants. When Stengel was not included with the starters when the manager split the squad, some sportswriters assumed he would not be with the team when the regular season began. Stengel, at McGraw's request, acted as a coach to the young players on the "B" squad, and worked hard, getting key hits in spring training games, and making the Giants as a reserve outfielder. McGraw and Stengel sometimes stayed up all night, discussing baseball strategy. With two Giant outfielders injured by June, Stengel was put in center field, and hit .368 in 84 games, as McGraw platooned him against right handers. If Stengel had enough plate appearances to qualify for the league batting championship, he would have finished second to Rogers Hornsby of the St. Louis Cardinals, who hit .401. The Giants played the Yankees again in the 1922 World Series; Stengel went 1–4 in Game 1, nursing an injured right leg muscle, which he aggravated running out an infield single in his first at bat in Game 2. Stengel was obviously limping when he was advanced to second base on another single, and McGraw sent in a pinch Runner. He did not use Stengel again in the Series, which the Giants won, four games to zero, with one tie. After the Series, Stengel and other major leaguers went on a barnstorming tour of Japan and the Far East.

On November 13, 1923, twenty-nine days after the World Series ended, Stengel was traded to the perennial second-division-dwelling Boston Braves with Dave Bancroft and Bill Cunningham for Joe Oeschger and Billy Southworth. Stengel had enjoyed his time in New York and was initially unhappy at the trade, especially since he had become close to McGraw. He soon cooled down, and later praised McGraw as "the greatest manager I ever played for". Despite nagging pains in his legs, Stengel played in 134 games during the 1924 season, the most since his days with the Dodgers, and hit .280 with a team-leading 13 stolen bases, but the Braves finished last. After the season, Stengel joined a baseball tour of Europe organized by McGraw and Charles Comiskey, with Edna Stengel accompanying the team. Casey Stengel met King George V and the Duke of York, later to become King George VI; Edna sipped tea with Queen Mary, an experience that strained her nerves. That winter, the Stengels moved into a house built by Edna's Father in Glendale, California, where they would live the rest of their lives. The marriage produced no children.

Stengel started 1925 on the active roster of the Braves, but was used as a pinch hitter and only started once, in right field against the Pirates on May 14, a game in which he got his only MLB hit of 1925, the final one of his major league career, a single. His average sank to .077. The owner of the Braves, Emil Fuchs purchased the Worcester Panthers of the Eastern League, and hired Stengel as player-manager and team President. Appel noted that in joining the Panthers, Stengel was "starting out on a managing career that would eventually take him to Cooperstown". With fans enjoying Stengel's on-field antics and his World Series heroics still recent, he was the Eastern League's biggest attendance draw. Between run-ins with the umpires, Stengel hit .302 in 100 games as the Panthers finished third.

The Panthers were not a box office success, and Fuchs planned to move the team to Providence, Rhode Island for the 1926 season, with Stengel to remain in his roles. However, McGraw, with whom Stengel had remained close, wanted him to take over as manager of the Giants' top affiliate, the Toledo Mud Hens of the American Association, at an increase in salary. He was still under contract, and Fuchs refused to release him. So Manager Stengel resigned, which President Stengel accepted, and the President then released Stengel as a player and resigned. Fuchs was outraged, and went to Judge Landis, who advised him to let the matter go.

Stengel had approached the new manager of the Dodgers, Max Carey, an old teammate from his time with the Pirates, seeking a job, and was hired as first base coach. Stengel thus ended an exile of seven years from the major leagues: Appel suggested that Stengel's reputation as a clown inhibited owners from hiring him although he was known as knowledgeable and able to manage the press. Sportswriter Dan Daniel described Stengel's hiring as "the return of an ancient Flatbush landmark", which might satisfy old Dodger fans upset at Robinson's dismissal at the end of 1931. Brooklyn finished third and then sixth with Stengel in the first base coaching box. Among the players was catcher Al Lopez; the two became close friends and would be managing rivals in the 1950s American League.

Stengel, hired only days before spring training, had but a limited opportunity to shape his new ballclub. The Dodgers quickly settled into sixth place, where they would end the season. Stengel, in later years, enjoyed discussing one 1934 game in Philadelphia's Baker Bowl: with Dodger pitcher Boom-Boom Beck ineffective, Stengel went out to the mound to take out the angry pitcher, who, instead of giving Stengel the baseball, threw it into right field, where it hit the metal wall with a loud noise. The right fielder, Hack Wilson, lost in thought, heard the boom, assumed the ball had been hit by a batter, and ran to retrieve it and throw it to the infield, to be met by general laughter. Stengel got the last laughs on the Giants as well. The Giants, six games ahead of the Cardinals on Labor Day, dissipated that lead, and with two games in the season were tied with St. Louis—with the final two games against Brooklyn at the Polo Grounds, games played in front of raucous crowds, who well remembered Terry's comments. Brooklyn won both games, while the Cardinals won their two games against the Cincinnati Reds, and won the pennant. Richard Bak, in his biography of Stengel, noted that the victories over the Terry-led Giants represented personal vindication for the Dodger manager.

The Dodgers finished fifth in 1935 and seventh in 1936. Dodger management felt Stengel had not done enough with the talent he had been given, and he was fired during the 1936 World Series between the Yankees and Giants. Stengel was paid for one year left on his contract, and he was not involved in baseball during the 1937 season. Stengel had invested in oil properties, as advised by one of his players, Randy Moore, a Texan; the investment helped make the Stengels well to do, and they put the profits in California real estate. Stengel considered going in the oil Business full-time, but Braves President Bob Quinn offered him the Boston managerial job in late 1937, and he accepted.

Stengel was both manager and an investor in the Braves. In his six years there, 1938 to 1943, his team never finished in the top half of the league standings, and the Boston club finished seventh four straight years between 1939 and 1942, saved from last place by the fact that the Phillies were even worse. The entry of the United States in World War II meant that many top players went into the Service, but for the Braves, the changeover made little difference in the standings. Among the young players to join the Braves was pitcher Warren Spahn, who was sent down to the minor leagues by the manager for having "no heart". Spahn, who would go on to a Hall of Fame career, much of it with the Braves, would play again for Stengel on the woeful New York Mets in 1965, and joke that he was the only person to play for Stengel both before and after he was a genius.

Stengel had been talked about as a possible Yankee manager in the early 1940s, but longtime team boss Ed Barrow (who retired in 1945) refused to consider it. Yankee manager Joe McCarthy retired during the 1946 season, and after the interim appointment of catcher Bill Dickey as player-manager, Bucky Harris was given the job in 1947. Stengel's name was mentioned for the job before Harris got it; he was backed by Weiss, but Larry MacPhail, then in charge of the franchise, was opposed. Harris and the Yankees won the World Series that year, but finished third in 1948, though they were not eliminated until the penultimate day. Harris was fired, allegedly because he would not give Weiss his home telephone number. MacPhail could no longer block Stengel's hiring, as he had sold his interest in the Yankees after the 1947 season. Weiss, who became general manager after MacPhail's departure, urged his partners in the Yankees to allow the appointment of Stengel, who aided his own cause by winning the championship with Oakland during the time the Yankee job stood open. Del Webb, who owned part of the Yankees, had initial concerns but went along. Yankee scouts on the West Coast recommended Stengel, and Webb's support helped bring around co-owner Dan Topping.

Stengel thought the 1943 season would be his last in baseball; Edna urged him to look after the family Business interests full-time, and Casey, who had always been an athlete, was reluctant to show himself at a baseball stadium with the imperfectly-healed injury. But early in the 1944 season, the minor-league Milwaukee Brewers had a managing vacancy to fill, as the Chicago Cubs had hired away the Brewer manager, Charlie Grimm, who had played with Stengel on the 1919 Pirates. Grimm told the Cubs he was obliged to see the Brewers had a competent replacement, and urged the Brewers to hire Stengel. The team owner, Bill Veeck, stationed with the Marines on Guadalcanal, thought ill of Stengel as a manager, and was very reluctant in his consent when reached by cable. Stengel was adept at fostering good relations with reporters, and the very talented team continued to win; by the end of May, Veeck had withdrawn his objections. The team won the American Association pennant, but lost in the playoffs to Louisville. Veeck, having returned to the United States, offered to rehire Stengel for 1945, but Stengel preferred another offer he received. This was from George Weiss of the New York Yankees, to manage the team Stengel had begun with, the Kansas City Blues, by then a Yankee farm club. Kansas City had finished last in the American Association as Milwaukee won the pennant, making it something of a comedown for Stengel, who hoped to return to the major leagues. Nevertheless, it was in his old home town, allowing him to see friends and relations, and he took the job. The Blues finished seventh in the eight-team league in 1945.

The Oaks finished second in the league behind the Seals in 1946, winning the first round of the playoffs against Los Angeles before losing to San Francisco in the finals. They finished fourth in 1947, beating San Francisco in the first round before losing to Los Angeles. Stengel managed the Oaks for a third year in 1948, with the roster heavy with former major leaguers. Among the younger players on the team was 20-year-old shortstop Billy Martin. Stengel was impressed by Martin's fielding, baseball acuity, and, when there were brawls on the field, fighting ability. The Oaks clinched the pennant on September 26, and defeated Los Angeles and the Seattle Rainiers to win the Governors' Cup. The Sporting News named Stengel the Minor League Manager of the Year.

Stengel was introduced as Yankee manager on October 12, 1948, the 25th anniversary of his second World Series home run to beat the Yankees. Joe DiMaggio attended the press conference as a sign of team support. Stengel faced obstacles to being accepted—Harris had been popular with the press and public, and the businesslike Yankee corporate culture and successful tradition were thought to be ill matched with a manager who had the reputation of a clown and who had never had a major league team finish in the top half of the standings. Stengel's MLB involvement had been in the National League, and there were several American League parks in which he had neither managed nor played.

During the 1949 season, Stengel had been more subdued than he usually was, but being the manager of the defending world champions and the Major League Manager of the Year relieved his inhibitions, and he was thereafter his talkative self. He also was more forceful in running the team, not always to the liking of veteran players such as DiMaggio and Phil Rizzuto. As part of his incessant shuffling of the lineup, Stengel had DiMaggio play first base, a position to which he was unaccustomed and refused to play after the first game. Stengel considered DiMaggio's decline in play as he neared the end of his stellar career more important than his resentment. In August, with the slugger in the middle of a batting slump, Stengel benched him, stating he needed a rest. DiMaggio returned after six games, and hit .370 for the rest of the season, winning the slugging percentage title. Other young players on the 1950 team were Billy Martin and Whitey Ford. The ascendence of these rookies meant more of the Yankees accepted Stengel's techniques, and diminished the number of those from the McCarthy era who resented their new manager. The Detroit Tigers were the Yankees' main competition, leading for much of the summer, but the Yankees passed them in the middle of September to win the pennant by three games. In the 1950 World Series, the Yankees played the Phillies, another of Stengel's old teams. The Yankees won in four games.

James noted that Stengel was not only the most successful manager of the 1950s, he was the most dominant manager of any single decade in baseball history. Stengel used his entire squad as Yankee manager, in contrast to other teams when he began his tenure, on which substitutes tended to get little playing time. Stengel's use of platooning meant more players saw more use, and he was generally more prone to put in a pinch hitter or replace his pitcher than other managers. Although only once in Stengel's time (1954) did the Yankees lead the league in number of pinch hitters, Stengel was known for using them in odd situations, once pinch-hitting for Moose Skowron in the first inning after a change of pitcher. Additionally, according to James, Stengel "rotated lineups with mad abandon, using perhaps 70 to 100 different lineups in a 154 game season". Although Stengel was not the first Yankee manager to use a regular relief pitcher—Harris had seen success with Joe Page in 1947—his adoption of the concept did much to promote it.

DiMaggio's retirement after the 1951 season gave Stengel greater control over the players, as the relationship between superstar and manager had sometimes been fraught. Stengel moved Mantle from right to center field in DiMaggio's place. The Yankees were not the favorites to win the pennant again in 1952, with DiMaggio retired, Mantle still recovering from his injury and several Yankees in military Service, deployed in the Korean War. Sportswriters favored the Indians. Younger players, some of whom Stengel had developed, came to the fore, with Martin the regular second baseman for the first time. Mantle proved slow to develop, but hit well towards the end of the season to finish at .311. The Yankee season was also slow to get on track; the team had a record of 18–17 on May 30, but they improved thereafter. Stengel prepared nearly 100 different lineup cards for the 1952 season. The race was still tight in early September, and on September 5, Stengel lectured the team for excessive levity on the train after the Yankees lost two of three to Philadelphia. Embarrassed by the episode, which made its way into print, the Yankees responded by winning 15 of their next 19, clinching the pennant on September 27.

The 1952 World Series was against the Brooklyn Dodgers, who by then had their stars of the 1950s such as Jackie Robinson, Gil Hodges and Duke Snider, with the team later nicknamed "The Boys of Summer". The teams split the first four games. With his pitching staff tired, Stengel gave the start to late-season acquisition Ewell Blackwell, who surrendered four runs in five innings, and the Yankees lost the game in 11 innings to the Dodgers behind Carl Erskine's complete game. Needing to win two in a row at Ebbets Field, Stengel pitched Raschi in Game 6, who won it with a save from Reynolds. With no tomorrow in Game 7, Stengel sent four pitchers to the mound. Mantle hit a home run to break a 2–2 tie, Martin preserved the lead by making a difficult catch off a Robinson popup, and Kuzava again secured the final outs for a Series victory, as the Yankees won 4–2 for their fourth straight World Series victory, matching the record set in 1936–1939, also by the Yankees.

The sportswriters had picked other teams to win the pennant in Stengel's first four years as Yankee manager; in 1953 they picked the Yankees, and this time they were proven correct. An 18-game winning streak in June placed them well in front, and they coasted to their fifth consecutive league championship, the first time a team had won five straight pennants. The Yankees played the Dodgers again in the World Series, which was less dramatic than the previous year's, as the Yankees won in six games, with Mantle, Martin, Berra and Gil McDougald—players developed under Stengel—taking the fore. The Yankees and Stengel won the World Series for the fifth consecutive year, the only team to accomplish that feat. Stengel, having taken the managerial record for consecutive pennants from McGraw (1921–1924) and McCarthy (1936–1939) and for consecutive titles from the latter, would say, "You know, John McGraw was a great man in New York and he won a lot of pennants. But Stengel is in town now, and he's won a lot of pennants too".

Stengel often rotated infielders between positions, with the Yankees having no real regular second baseman or shortstop between 1954 and 1958. Despite this, the Yankees had a strong defensive infield throughout. Stengel gave great attention to the double play, both defensively and in planning his lineup, and the Yankees responded by being first in the league in double plays as a defense six times in his twelve-year tenure, and the batters hit into the fewest double plays as a team eight times in that era. Having had few players he could rely upon while managing Brooklyn and Boston, Stengel treated his Yankee roster with little sentimentality, trading players quickly when their performance seemed to decline, regardless of past accomplishments. Assured of quality replacements secured by the Yankee front office, the technique worked well, but was not a success with the Mets, where no quality replacements were available, and the technique caused confusion and apathy among the players.

A child of the Jim Crow era, and from a border state (Missouri) with southern characteristics, Stengel has sometimes been accused of being a racist, for Example by Roy Campanella Jr., who stated that Stengel made racist remarks from the dugout during the World Series towards his Father Roy Campanella, Jackie Robinson and other black stars of the Dodgers. Bak noted, though, that Stengel was a "vicious and inventive" bench jockey, hazing the other team with whatever might throw off their performance. Stengel had poor relations with Robinson; each disliked the other and was a vocal critic. One widely quoted Stengel comment was about catcher Elston Howard, who became the first black Yankee in 1955, eight years after Robinson had broken the color barrier, "they finally get me a nigger, I get the only one who can't run". Howard, though, denied that Stengel had shouted racial epithets at the Dodgers, and said "I never felt any prejudice around Casey". Al Jackson, a black pitcher with the Mets under Stengel, concurred, "He never treated me with anything but respect". According to Bill Bishop in his account of Stengel, "Casey did use language that would certainly be considered offensive today, but was quite Common vernacular in the fifties. He was effusive in his praise of black players like Satchel Paige, Larry Doby and Howard". Conscious of changing times, Stengel was more careful in his choice of words while with the Mets.

Stengel was sometimes considered thoughtless or even cruel towards his players. Examples of this include his playing Joe DiMaggio at first base, and sometimes batting Phil Rizzuto ninth, behind the pitcher, as well as his dismissal of the shortshop on Old-Timers' Day in 1956. In spite of their falling out over the 1957 trade, Billy Martin, by then manager of the Yankees, wore a black armband in remembrance of Stengel during the 1976 season, the sole Yankee to do so. According to Creamer, "It doesn't seem to be stretching the point too far to say that Ned Hanlon begat John McGraw who begat Casey Stengel who begat Billy Martin".

What was seen as a failure to keep discipline on the team hurt Stengel's standing with the Yankee owners, Topping, Weiss and Webb, as did the defeat in the 1957 Series. Stengel was by then aged 67, had several times fallen asleep in the dugout, and players complained that he was growing more irritable with the years. Former Yankee catcher Ralph Houk, who had been successful as a minor league manager and was Stengel's first base coach, was seen by ownership as the next manager of the Yankees. Stengel's contract, his fifth two-year deal, was up after the 1958 season. As ownership debated whether to renew it, the Yankees led by as many as 17 games, and won the pennant by 10. The Braves were the opponent in the 1958 World Series. The Braves won three of the first four games, but the Yankees, backed by the pitching of Bob Turley (who got two wins and a save in the final three games) stormed back to win the Series. Firing Stengel under such circumstances was not possible, and ownership gave him another two-year contract, to expire after the 1960 season.

After the 1959 season, Weiss made a trade with Kansas City to bring Roger Maris to the Yankees. Stengel was delighted with the acquisition and batted Maris third in the lineup, just in front of Mantle, and the new Yankee responded with an MVP season in 1960. Stengel had health issues during the season, spending ten days in the hospital in late May and early June, with the illness variously reported as a bladder infection, a virus or influenza. The Yankees were challenged by the Baltimore Orioles for most of the year but won the pennant by eight games, Stengel's tenth as manager, tying the major league record held by McGraw. The Yankees played the Pittsburgh Pirates, again a team Stengel had played for, in the 1960 World Series, and Stengel picked Art Ditmar, who had won the most games, 15, for the Yankees, rather than the established star, Whitey Ford. The Pirates knocked Ditmar out of the box in the first inning, and won Game 1. Ditmar was also knocked out of Game 5; Ford won Games 3 and 6 of the seven-game series with shutouts, but could not pitch Game 7, as he might have if Stengel had used him in Games 1 and 5. The Pirates defeated the Yankees in Game 7, 10–9, on a ninth-inning Bill Mazeroski home run.

Shortly after the Yankees returned to New York, Stengel was informed by the team owners that he would not be given a new contract. His request, that the termination be announced at a press conference, was granted and on October 18, 1960, Topping and Stengel appeared before the microphones. After Topping evaded questions from the press about whether Stengel had been fired, Stengel took the microphone, and when asked if he had been fired, stated, "Quit, fired, whatever you please, I don't care". Topping stated that Stengel was being terminated because of his age, 70, and alleged that this would have happened even had the Yankees won the World Series.

As part of baseball's expansion in the early 1960s, a franchise was awarded to New York, to play in the National League beginning in 1962, and to be known as the New York Mets. It was hoped that the new team would be supported by the many former Giant and Dodger fans left without a team when the franchises moved to California after the 1957 season. There were rumors through the 1961 season that Stengel would be the manager, but he initially showed no interest in managing a team that, given the rules for the expansion draft, was unlikely to be competitive. George Weiss had been forced out as Yankee general manager and hired by the Mets. He wanted Stengel as manager, and after talks with the Mets principal owners, Joan Whitney Payson and M. Donald Grant, Stengel was introduced as Mets manager at a press conference on October 2, 1961. Leonard Koppett of The New York Times suggested that Stengel took the job to give something back to the game that had been his life for half a century.

The tributes to Stengel upon his death were many. Maury Allen wrote, "He is gone and I am supposed to cry, but I laugh. Every time I saw the man, every time I heard his voice, every time his name was mentioned, the creases in my mouth would give way and a smile would come to my face". Richie Ashburn, a member of the 1962 Mets, stated, "Don't shed any tears for Casey. He wouldn't want you to ... He was the happiest man I've ever seen". Jim Murray of the Los Angeles Times wrote, "God is certainly getting an earful tonight".

The 1963 season unfolded for the Mets much like the previous year's, though they lost only eight games to begin the season, rather than nine, but they still finished 51–111, in last place. One highlight, though it did not count in the standings, was the Mayor's Trophy Game on June 20 at Yankee Stadium. Stengel played to win; the Yankees under Houk possibly less so, and the Mets beat the Yankees, 6–2.

In 1964, the Mets moved into the new Shea Stadium; Stengel commented that "the park is lovelier than my team". The Mets finished 53–109, again in last place. By this time, the fans were starting to be impatient with the losing, and a number of people, including Sportscaster Howard Cosell and former Dodger Jackie Robinson, criticized Stengel as ineffective and prone to fall asleep on the bench. Stengel was given a contract for 1965, though Creamer suggested that Weiss, Grant and Payson would have preferred that the 74-year-old Stengel retire.

The Mets retired Stengel's uniform number, 37, on September 2, 1965, after which he returned to his home in California. He was kept on the team payroll as a vice President, but for all intents and purposes he was out of baseball. His life settled into a routine of attending the World Series (especially when in California), the All-Star Game, Mets spring training, and the baseball writers' dinner in New York. The Writers, who elect members of baseball's Hall of Fame, considered it unjust that Stengel should have to wait the usual five years after retirement for election, and waived that rule. On March 8, 1966, at a surprise ceremony at the Mets spring training site in St. Petersburg, Stengel was told of his election; he was inducted in July along with Ted Williams. Thereafter, he added the annual Hall of Fame induction ceremony to his schedule. In 1969, the "Amazin' Mets" justified their nickname by unexpectedly winning the World Series over the favored Orioles. Stengel attended the Series, threw out the first ball for Game 3 at Shea, and visited the clubhouse after the Mets triumphed in Game 5 to win the Series. The Mets presented him with a championship ring.

Bill Veeck said of Stengel in 1966, soon after the manager's retirement, "He was never necessarily the greatest of managers, but any time he had a ball club that had a chance to win, he'd win". Stengel's American League rival, Al Lopez, once said of him "I swear I don't understand some of the things he does when he manages". Though platooning survives, Stengel's intuitive approach to managing is no longer current in baseball, replaced by the use of statistics, and the advent of instant replay makes obsolete Stengel's tendency to charge from the dugout to confront an Umpire over a disputed call.

Stengel also participated in Old-Timers' Day at a number of ballparks, including, regularly, Shea Stadium. In 1970, the Yankees invited him to Old-Timers' Day, at which his number, 37, was to be retired. By this time, the Yankee ownership had changed, and the people responsible for his dismissal were no longer with the team. He accepted and attended, and Stengel became the fifth Yankee to have his number retired. He thereafter became a regular at the Yankees' Old-Timers' Day.

By 1971, Edna Stengel was showing signs of Alzheimer's disease, and in 1973, following a stroke, she was moved into a nursing home. Casey Stengel continued to live in his Glendale home with the help of his housekeeper June Bowdin. Stengel himself showed signs of senility in his last years, and during the final year of his life, these increased. In his last year, Stengel cut back on his travel schedule, and was too ill to attend the Yankees' Old-Timers Day game in August 1975, at which it was announced that Billy Martin would be the new team manager. A diagnosis of cancer of the lymph glands had been made, and Stengel realized he was dying. In mid-September, he was admitted to Glendale Memorial Hospital, but the cancer was inoperable. He died there on September 29, 1975. Stengel was interred in the Forest Lawn Memorial Park Cemetery in Glendale.

As part of professional baseball's centennial celebrations in 1969, Stengel was voted its "Greatest Living Manager". He had his uniform number, 37, retired by both the Yankees and the Mets. He is the first man in MLB history to have had his number retired by more than one team based solely upon his managerial accomplishments, and was joined in that feat by the late Sparky Anderson in 2011, who had called Stengel "the greatest man" in the history of baseball. The Yankees dedicated a plaque in Yankee Stadium's Monument Park in Stengel's memory on July 30, 1976, reading: "Brightened baseball for over 50 years; with spirit of eternal youth; Yankee manager 1949–1960 winning 10 pennants and 7 world championships including a record 5 consecutive, 1949–1953". He was inducted into the New York Mets Hall of Fame in 1981.

Edna Stengel died in 1978, and was interred next to her husband. In addition to the marker at their graves in Forest Lawn Cemetery, there is a plaque nearby in tribute to Casey Stengel, which besides biographical information contains a bit of Stengelese, "There comes a time in every man's life, and I've had plenty of them".

Stengel is the only man to have worn the uniform (as player or manager) of all four Major League Baseball teams in New York City in the 20th century: the Dodgers, Giants, Yankees and Mets. He is the only person to have played or managed for the home team in five New York City major league venues: Washington Park, Ebbets Field, the Polo Grounds, Shea Stadium and the original Yankee Stadium. In 2009, in an awards segment on the MLB Network titled "The Prime 9", he was named "The Greatest Character of The Game", beating out Yogi Berra.

Stengel tried incessantly to promote the Mets, talking to reporters or anyone else who would Listen. He often used the word "amazin' " (as he put it) and soon this became the "Amazin' Mets", a nickname that stuck. Stengel urged the fans, "Come out and see my amazin' Mets. I been in this game a hundred years but I see new ways to lose I never knew existed before". Stengel was successful in selling the team to some extent, as the Mets drew 900,000 fans, half again as many as the Giants had prior to their departure, though the games against the Giants and Dodgers accounted for half of the total. The team was less successful on the field, finishing with a record of 40–120, the most losses of any 20th century major league team. They finished 60​⁄₂ games behind the pennant-winning Giants, and 18 games behind ninth-place Chicago.

Stengel has been praised for his role in successfully launching the Mets. Appel deemed the Mets' beginnings unique, as later expansion teams have been given better players to begin with, "few expansion teams in any sport have tried the formula—a quotable, fan-popular man who would charm the press and deflect attention away from ineptness on the field". Arthur Daley of The New York Times wrote, "he gave the Mets the momentum they needed when they needed it most. He was the booster that got them off the ground and on their journey. The smoke screen he generated to accompany the blast-off obscured the flaws and gave the Mets an acceptance and a following they could not have obtained without him". James wrote, "Stengel became such a giant character that you really can't talk about him in the past. He became an enduring part of the game". Commissioner william Eckert said of Stengel, "he's probably done more for baseball than anyone".