On April 17, Jamie Moyer became the oldest pitcher in MLB history to earn a win. The previous record was held by Jack Quinn, who earned his last win in 1932, two months after his 49th birthday. Moyer became the oldest player in MLB history to record an RBI on May 16 when he singled in two runs in the 4th inning against the Arizona Diamondbacks. He also extended his record for the oldest pitcher to record a win. Five days later, Moyer started for the Rockies against the Marlins at Marlins Park. This appearance represented the 50th MLB stadium Moyer has pitched in, the most of any pitcher to debut since 1900. Target Field in Minnesota is the only active stadium in which Moyer has not pitched.

Moyer pitched at Saint Joseph's University, where in 1984 he set the school's single-season records in wins, with 16, earned run average (ERA), with 1.99, and strikeouts, with 90. In 1997 he became the only Saint Joseph's baseball player to have his jersey number, number 10, retired, and was one of three inductees into the first class of the St. Joseph's Baseball Hall of Fame.

The Chicago Cubs selected Moyer in the sixth round of the 1984 amateur draft. Moyer was selected a New York–Penn League All-Star in 1984. He made his major league debut with the Cubs on June 16, 1986, against Steve Carlton and the Philadelphia Phillies, and earned his first win. Later that year, on August 16, he threw his first shutout against the Montreal Expos. He was also the starting pitcher for the Cubs on the day that Greg Maddux made his major league debut.

In 1987, Moyer ranked tenth in the National League in strikeouts with 147, while winning 12 games.

Following his then-best season in 1988, he was traded to the Texas Rangers as part of the 9-player Rafael Palmeiro for Mitch Williams trade.

Moyer was on the disabled list with a sore left shoulder for much of a disappointing 1989 season. 1990 saw Moyer spend time in the bullpen before regaining a spot in the starting rotation.

Moyer was released as a free agent after the 1990 season and was signed by the St. Louis Cardinals. He made seven starts for the Cardinals in 1991 before being sent to the minor leagues on May 24, and was released on October 14.

On December 18, 1992, Moyer signed with the Baltimore Orioles.

Moyer began the 1993 season in the Oriole minor leagues, before being called up on May 30. He tied his career-high total in wins with 12 and a new career-low ERA of 3.43. The strike-shortened 1994 season was disappointing for him, but he was third on the staff in innings pitched. In 1995, Moyer again found himself in the Baltimore bullpen, but worked his way back into the starting rotation.

Moyer, after spending many of his playing years living in Seattle, moved to Florida with his wife Karen (the daughter of former Notre Dame basketball coach and retired ESPN Sportscaster Digger Phelps) and their eight children. The Moyers' two youngest daughters were adopted from Guatemala. In 1996, Moyer earned a Bachelor of General Studies degree from Indiana University.

In 1997, Moyer was fifth in the American League with 17 wins. His 17–5 record gave him the second-highest winning percentage (.773) in the league. Moyer made his first postseason start against his former club Baltimore, but was forced out with a strained elbow in the fifth inning.

In 1998, Moyer went 15–9 with a 3.53 ERA. He was third in innings pitched with 234.1. He registered his 100th career win against the Cleveland Indians on August 27, as well as his 1000th career strikeout with a sixth inning strikeout of David Bell. He was named Seattle's Pitcher of the Year by the Seattle chapter of the BBWAA.

In 1999, Moyer went 14–8 with a 3.87 ERA and was voted to The Sporting News AL All-Star team. He again won the Seattle Pitcher of the Year award.

2000 saw Moyer rebound from an early shoulder injury to tally 13 wins, giving him at least 13 in each of his past five seasons. He made his first Opening Day start for Seattle, but lost to the Boston Red Sox 2–0 on April 4. His shoulder problems led his ERA to balloon to 5.49. A knee injury suffered on the last pitch of a simulated game caused him to miss Seattle's trip to the American League Championship Series against the eventual World Series champion New York Yankees.

In 2001 Moyer won 20 games, ranked tied for second in the American League, and his 3.43 ERA was sixth in the AL. He earned his 150th career win against the Texas Rangers on September 24. He became only the second Mariner in history to win 20 games on October 5, former teammate Randy Johnson being the other. Moyer went 3–0 with a 1.89 ERA in the postseason. He won Games 2 and 5 for the Mariners against the Cleveland Indians and also carried Game 3 against the New York Yankees before Seattle lost in Game 5.

In 2002, Moyer went 13–8 with a then career low 3.32 ERA. Although he pitched 20 more innings and had a lower ERA than in 2001, he won seven fewer games.

In 2003, Moyer won a career high 21 games, lost 7, and had a career low 3.27 ERA. He tied for second in the American League for wins and was sixth in ERA. His .750 winning percentage placed him fourth in the league and his 21 wins are a club record. He became the only Seattle pitcher to win 20 games more than once. Moyer was voted to his first and only All-Star Game in 2003. He was named for the third time the Seattle Pitcher of the Year. Moyer was also the winner of the Roberto Clemente Award, given annually to the big leaguer whose success on the field is mirrored by his impact in community Service, The Hutch Award, presented annually by the world-renowned Fred Hutchinson Cancer Research Center to an MLB player displaying "honor, courage and dedication to baseball, both on and off the field", and The Lou Gehrig Award, presented annually to the MLB player who both on and off the field best exemplifies the character of Lou Gehrig.

In 2004, Moyer went 7–13 and posted his first losing record since 1994. His ERA was 5.21 and he surrendered an MLB-leading 44 home runs. While the year started well for him, going 5–0 with a 1.59 ERA from May 20 – June 18, Moyer ended 2004 on a 10-game losing streak. He threw the slowest fastball of all AL starters, averaging 81.6 mph. One positive for Moyer was he was awarded the Branch Rickey Award for his exceptional community Service following the season.

During the 2005 season Moyer passed Randy Johnson to become the winningest pitcher for the Mariners on May 30. On July 8, 2005, Moyer became the 25th southpaw to win 200 games in Major League Baseball. He finished with a 13–7 record and for the second year in a row he threw the slowest fastball of all major league starters, averaging 81.7 mph.

On August 19, 2006, Moyer was traded to the Philadelphia Phillies for minor league pitchers Andrew Barb and Andrew Baldwin. In his first start with the Phillies, Moyer set a franchise record as the oldest pitcher to record a win. In eight starts with the Phillies in 2006, Moyer went 5–2 with a 4.03 ERA. After the season, Moyer signed a two-year extension worth $10.5 million with the Phillies on October 23.

He threw the slowest fastball of all NL starters in 2007, averaging 81.1 miles per hour (130.5 km/h).

On October 25, Moyer made his first World Series start against the Tampa Bay Rays, pitching 6​⁄₃ innings, giving up three runs, and receiving a no-decision. More impressive was that he pitched with a severe stomach virus. He won his first World Series ring when the Phillies defeated the Rays on October 29 in his 23rd Major League Season. Following the game, the pitchers mound at Citizens Bank Park was dug up and given to Moyer by his teammates. In his speech at the World Series celebration at Citizens Bank Park on October 31, he related to the fans that he grew up as a Phillies fan and played hooky from Souderton Area High School to attend the Phillies' championship parade in 1980. On December 15, 2008 Moyer signed a two-year, $16 million contract with the Phillies, keeping him with the club through the conclusion of the 2010 season.

Moyer's oldest son Dillon was drafted in the 22nd round of the 2010 MLB draft by the Minnesota Twins but did not sign and instead attended UC Irvine for two years. He transferred to UC San Diego for his Junior, and final, year, as he graduated in three years with a major in Sociology and minor in Education. He was later drafted in the 38th round of the 2013 MLB draft by the Los Angeles Dodgers and did sign with them. Another son, Hutton, was drafted in the seventh round of the 2015 MLB draft.

On January 18, 2012, Moyer signed a minor-league contract (with an invitation to spring training) with the Colorado Rockies, a team that did not exist when he made his MLB debut in 1986. The March 30 NBC Nightly News reported that Moyer made the opening day roster for the Rockies, and would be the number-two starter in the rotation. The report was confirmed the following day by the Associated Press. He made his Rockies debut on April 7 against the Houston Astros. He pitched 5 innings, giving up three runs, and received the loss.

Moyer's pitching approach evolved as he aged. Most pitchers lose velocity later in their career, and Moyer was no exception – his average fastball speed in 2012 was about 80 MPH, a very slow speed for a non-knuckleball pitcher. Instead of velocity, Moyer relied on control and mixing his pitches. He threw five main pitches: a sinker, a cut fastball, a slider, a changeup, and a curveball.

On February 11, 2014, Moyer and fellow former Phillie Matt Stairs were announced to join the Phillies' television broadcast crew as color analysts for the 2014 season. Moyer and Stairs joined in-game reporter Gregg Murphy and play-by-play voice Tom McCarthy. Following the season, Moyer announced he would not return to the booth in 2015, citing a Desire to spend more time with his family.

In Moyer's bullpen debut on August 18, he relieved Martínez in the fourth inning after a rain delay, pitching six scoreless innings to earn his 11th win of the season. A similar situation occurred on August 28, as Moyer again relieved Martinez in the third inning after a rain delay. He pitched four and one third innings in relief, giving up one earned run and picked up his 12th win of the season.