Stevenson was raised in the city of Bloomington, Illinois; his family was a member of Bloomington's upper class and lived in one of the city's well-to-do neighborhoods. On December 30, 1912, at the age of twelve, Stevenson accidentally killed Ruth Merwin, a 16-year-old friend, while demonstrating drill technique with a rifle, inadvertently left loaded, during a party at the Stevenson home. Stevenson was devastated by the accident and rarely referred to it as an adult. However, in 1955 Stevenson heard about a woman whose son had experienced a similar tragedy. He wrote to her that she should tell her son that "he must live for two", which Stevenson's friends took to be a reference to the shooting incident.

Stevenson left Bloomington High School after his junior year and attended University High School in Normal, Illinois, Bloomington's "twin city", just to the north. He then went to boarding school in Connecticut at The Choate School (now Choate Rosemary Hall), where he played on the tennis team, acted in plays, and was elected editor-in-chief of The News, the school newspaper. Upon his graduation from Choate in 1918, he enlisted in the Navy and served at the rank of Seaman Apprentice, but his training was completed too late for him to participate in World War I.

He attended Princeton University, becoming managing Editor of The Daily Princetonian, a member of the American Whig-Cliosophic Society, a member of the Quadrangle Club, and received a B.A. degree in 1922 in literature and history. Under prodding from his father he then went to Harvard Law School, but found the law to be "uninteresting", and withdrew after failing several classes. He returned to Bloomington where he wrote for the family newspaper, The Daily Pantagraph, which was founded by his maternal great-grandfather Jesse Fell. The Pantagraph, which had one of the largest circulations of any newspaper in Illinois outside of the Chicago area, was a main source of the Stevenson family's wealth.

A year after leaving Harvard, Stevenson became interested in the law again after talking to Supreme Court Justice Oliver Wendell Holmes Jr. When he returned home to Bloomington, he decided to finish his degree at Northwestern University School of Law, attending classes during the week and returning to Bloomington on the weekends to write for the Pantagraph. Stevenson received his J.D. degree from Northwestern in 1926 and passed the Illinois State Bar examination that year. He obtained a position at Cutting, Moore & Sidley, an old and conservative Chicago law firm.

In 1928, Stevenson married Ellen Borden, a well-to-do socialite. The young couple soon became popular and familiar figures on the Chicago social scene; they especially enjoyed attending, and hosting, costume parties. They had three sons: Adlai Stevenson III, who would become a U.S. Senator; Borden Stevenson, and John Fell Stevenson. In 1935, Adlai and Ellen purchased a 70-acre (28 ha) tract of land along the Des Plaines River near Libertyville, Illinois, a wealthy suburb of Chicago. They built a home on the property and it served as Stevenson's official residence for the rest of his life. Although he spent relatively little time there due to his career, Stevenson did consider the farm to be his home, and in the 1950s, he was often called "The Man from Libertyville" by the national news media. Stevenson also purchased a farm in northwestern Illinois, just outside Galena, where he frequently rode horses and kept some cattle.

In July 1933, Stevenson took a job opportunity as special attorney and assistant to Jerome Frank, the general counsel of the Agricultural Adjustment Administration (AAA), a part of President Franklin D. Roosevelt's New Deal. Following the repeal of Prohibition in December 1933, Stevenson changed jobs, becoming chief attorney for the Federal Alcohol Control Administration (FACA), a subsidiary of the AAA which regulated the activities of the alcohol industry.

In 1935, Stevenson returned to Chicago to practice law. He became involved in civic activities, particularly as chairman of the Chicago branch of the Committee to Defend America by Aiding the Allies from 1940 to 1941. As chairman, Stevenson worked to raise public support for military and economic aid to the United Kingdom and its allies in fighting Nazi Germany during the Second World War. Stevenson "believed Britain [was] America's first line of defense" and "argued for a repeal of the neutrality legislation" and support for President Roosevelt's Lend-Lease programme. His efforts earned strong criticism from Colonel Robert R. McCormick, the powerful, isolationist publisher of the Chicago Tribune, and a leading member of the non-interventionist America First Committee.

The Adlai E. Stevenson II Farm in Mettawa, Illinois, which was Stevenson's home from 1936 to 1965, is on the National Register of Historic Places and has been designated a National Historic Landmark.

In 1940, Colonel Frank Knox, newly appointed by President Franklin D. Roosevelt as Secretary of the Navy, offered Stevenson a position as Principal Attorney and special assistant. In this capacity, Stevenson wrote speeches, represented Secretary Knox and the Navy on committees, toured the various theaters of war, and handled many administrative duties. Since Knox was largely a figurehead, there were few major roles for Stevenson. However, in early 1944 he joined a mission to Sicily and Italy for the Foreign Economic Administration to report on the country's economy. After Knox died in April 1944, Stevenson returned to Chicago where he attempted to purchase Knox's controlling interest in the Chicago Daily News, but his syndicate was outbid by another party.

In a parallel universe featured in the Sliders Season Five episode "The Return of Maggie Beckett", the German Wehrmacht broke through the Allied lines at the Battle of the Bulge in 1944, which caused World War II to drag on until 1947. General Eisenhower was relieved as the Supreme Commander of the Allied Forces in Europe and returned to the United States in disgrace. Consequently, Stevenson became President. The Stevenson administration made the Roswell UFO incident in July 1947 public knowledge and signed the Reticulan-American Free Trade Agreement (RAFTA), giving the US access to advanced Reticulan Technology. This leads to a manned mission to Mars in the 1990s.

In 1945, Stevenson took a temporary position in the State Department, as special assistant to US Secretary of State Edward Stettinius to work with Assistant Secretary of State Archibald MacLeish on a proposed world organization. Later that year, he went to London as Deputy United States Delegate to the Preparatory Commission of the United Nations Organization, a position he held until February 1946. When the head of the delegation fell ill, Stevenson assumed his role. His work at the Commission, and in particular his dealings with the representatives of the Soviet Union, resulted in appointments to the US delegations to the United Nations in 1946 and 1947.

In 1948, Stevenson was chosen by Jacob Arvey, the leader of the powerful Chicago Democratic political organization, to be the Democratic candidate in the Illinois gubernatorial race against the incumbent Republican, Dwight H. Green. In a surprise upset, Stevenson defeated Green by 572,067 votes, a record margin in Illinois gubernatorial elections. President Truman carried Illinois by only 33,612 votes against his Republican opponent, Thomas E. Dewey, leading some commentators to write that "Clearly, Adlai had carried the President in with him." Paul Douglas, a University of Chicago professor of economics, was elected Senator on the same ticket.

On June 2, 1949, Stevenson privately gave a sworn deposition as a character witness for Alger Hiss, a former State Department official who was later found to be a spy for the Soviet Union. Stevenson had infrequently worked with Hiss, first in the legal division of the AAA in 1933, and then in 1945, 1946, and 1947 on various United Nations projects, but he was not a close friend or associate of him. In the deposition, Stevenson testified that the reputation of Hiss for integrity, loyalty, and veracity was good. In 1950 Hiss was found guilty of perjury on the spying charges. Stevenson's deposition, according to his biographer Porter McKeever, would later be used in the 1952 presidential campaign by Senators Joseph McCarthy and Richard Nixon to "inflame public opinion and attack Adlai as 'soft on communism'." In the 1952 campaign, Senator Nixon would claim that Stevenson's "defense of Hiss" reflected such "poor judgment" on his part that "doubt was cast about Adlai's capacity to govern." In a 1952 appearance on NBC's Meet the Press, Stevenson responded to a question about his deposition for Hiss by saying "I'm a Lawyer. I think that one of the most fundamental responsibilities...particularly of lawyers, is to give testimony in a court of law, to give it honestly and willingly, and it will be a very unhappy day for Anglo-Saxon justice when a man, even in public life, is too timid to state what he knows and what he has heard about a defendant in a Criminal trial for fear that defendant might later be convicted. That would to me be the ultimate timidity."

Principal among Stevenson's achievements as Illinois governor were reforming the state police by removing political considerations from hiring practices and instituting a merit system for employment and promotion, cracking down on illegal gambling, and improving the state highways. He sought, with mixed success, to cleanse the Illinois state government of corruption; in one instance he fired the warden of the state penitentiary for overcrowding, political corruption, and incompetence that had left the prisoners on the verge of revolt, and in another instance Stevenson fired the superintendent of an institution for alcoholics when he learned that the superintendent, after receiving bribes from local tavern owners, was allowing the patients to buy drinks at local bars. Two of Stevenson's major initiatives as governor were a proposal to create a constitutional convention (called "con-con") to reform and improve the Illinois state constitution, and several crime bills that would have provided new resources and methods to fight Criminal activities in Illinois. Most of the crime bills and con-con failed to pass the state legislature, much to Stevenson's chagrin. However, Stevenson did agree to support a Republican alternative to con-con called "Gateway", it passed the legislature and was approved by Illinois voters in a 1950 referendum. Stevenson's push for an improved state constitution "began the process of constitutional change...and in 1969, four years after his death, the goal was achieved. It was perhaps his most important achievement as governor." The new Constitution had the effect of removing the structural limitations on the growth of government in the State.

In the alternate history short story "The Impeachment of Adlai Stevenson" by David Gerrold included in the anthology Alternate Presidents, Stevenson is elected in 1952 after Dwight D. Eisenhower makes the mistake of accepting Joseph McCarthy as his running mate instead of Richard Nixon. He successfully runs for re-election in 1956, once again defeating General Eisenhower. However, he proves to be an extremely unpopular President.

In the alternate history novel Dominion by C. J. Sansom, World War II ends in June 1940 when the British government, under the leadership of the Prime Minister Lord Halifax, signs a peace treaty with Nazi Germany in Berlin. Franklin D. Roosevelt is steadfast in his opposition to the Nazis and the treaty, which results in him losing the 1940 election to his Republican opponent, Robert A. Taft, who becomes the 33rd President. Taft is re-elected in 1944 and 1948 but Stevenson defeats him in 1952, becoming the 34th President. Shortly after Stevenson's election in November 1952, The Times, which is owned by the pro-Nazi British Prime Minister Lord Beaverbrook, speculates that Stevenson will follow in Roosevelt's footsteps and pursue an interventionist foreign policy regarding European affairs. Several weeks later, President-elect Stevenson gives a speech indicating that he intends to begin trading with the Soviet Union upon taking office on January 20, 1953.

In Billy Wilder's 1954 romantic comedy film Sabrina, the Larrabee family are millionaires who live on Long Island, New York. During a party, Oliver Larrabee (played by Walter Hampden) takes his younger son, David (played by william Holden), to task for romancing the family chauffeur's soignée daughter, Sabrina (played by Audrey Hepburn), and neglecting his fiancée. Also present is Oliver's older son, Linus (played by Humphrey Bogart). Oliver to David: "I'll overlook for the moment the fact that you're an engaged man and merely remind you of your marital record to date. First, that Hungarian Countess, who only married you to bring her family over: her mother, her father and five brothers—all of them badly in need of costly dental repairs.... Then that Twyman girl—her family 50 years on the social register, and she has the audacity to wear on her wedding dress not a corsage but the Stevenson button!" Linus: "Father, you promised not to swear."

Unlike 1952, Stevenson was an announced, active candidate for the Democratic presidential nomination in 1956. Initially, with polls showing Eisenhower headed for a landslide re-election, few Democrats wanted the 1956 nomination, and Stevenson hoped that he could win the nomination without a serious contest, and without entering any presidential primaries. However, on September 24, 1955, Eisenhower suffered a serious heart attack. Although he recovered and eventually decided to run for a second term, concerns about his health led two prominent Democrats, Tennessee Senator Estes Kefauver and New York Governor Averell Harriman, to decide to challenge Stevenson for the Democratic nomination. After being told by his aides that he needed to enter and win several presidential primaries to defeat Kefauver and Harriman, Stevenson entered the New Hampshire, Minnesota, Florida, and California primaries. He won New Hampshire, but was upset by Kefauver in the Minnesota primary, who successfully portrayed Stevenson as a "captive" of corrupt Chicago political bosses and "a corporation Lawyer out of step with regular Democrats." Stevenson next battled Kefauver in the Florida primary, where he agreed to debate Kefauver on radio and television. Stevenson later joked that in Florida he had appealed to the state's citrus farmers by "bitterly denouncing the Japanese beetle and fearlessly attacking the Mediterranean fruit fly." He narrowly defeated Kefauver in Florida by 12,000 votes, and then won the California primary over Kefauver with 63% of the vote, effectively ending Kefauver's presidential bid.

In Michael P. Kube-McDowell's alternate history novel Alternities, Stevenson is mentioned as having been elected President in 1956 and serving for two terms, though he is quoted as describing his second term as a curse.

Early in 1957, Stevenson resumed law practice, allying himself with Judge Simon H. Rifkind to create a law firm based in Washington, D.C. (Stevenson, Paul, Rifkind, Wharton & Garrison), and a second firm in Chicago (Stevenson, Rifkind & Wirtz). Both law firms were related to New York City's Paul, Weiss, Rifkind, Wharton & Garrison. Stevenson's associates in the new law firm included Willard Wirtz, william McCormick Blair Jr., and Newton N. Minow; each of these men would later serve in the Kennedy and Johnson administrations. He also accepted an appointment, along with other prominent Democrats, to the new Democratic Advisory Council, which "pursued an aggressive line in attacking the [Republican] Eisenhower administration and in developing new Democratic policies." He was also employed part-time by the Encyclopædia Britannica as a legal consultant.

Against the advice of many of his political advisers, Stevenson insisted on calling for an international ban to aboveground nuclear weapons tests, and for an end to the military draft. Despite strong criticism from President Eisenhower and other leading Republicans, such as Vice-President Nixon and former New York Governor Thomas Dewey, that his proposals were naive and would benefit the Soviet Union in the cold war, Stevenson held his ground, saying in various speeches that "Earth's atmosphere is contaminated from week to week by exploding hydrogen bombs...We don't want to live forever in the Shadow of a radioactive mushroom cloud...[and] growing children are the principal potential sufferers" of increased strontium 90 in the atmosphere. In the end, Stevenson's push to ban atmospheric nuclear bomb tests "cost him dearly in votes", yet "Adlai finally won the verdict", as Eisenhower suspended aboveground nuclear tests in 1958, President Kennedy would sign the Partial Nuclear Test Ban Treaty into law in 1963, and President Nixon would end the military draft in 1973.

The Writer Gore Vidal, who admired and supported Stevenson, based a main character in his 1960 Broadway play The Best Man on Stevenson. The play, which was nominated for six Tony Awards, centers on the contest for the presidential nomination at a fictitious political convention. One of the main contenders for the nomination is Secretary of State william Russell, a principled, liberal intellectual. The character is based on Stevenson; his main opponent is the ruthless, unscrupulous Senator Joseph Cantwell, whom Vidal modeled on Richard Nixon and the Kennedy brothers. The play was turned into a 1964 film of the same name, with actor Henry Fonda playing Russell. Fonda had been a Stevenson supporter at the 1960 Democratic National Convention.

In April 1961 Stevenson suffered the greatest humiliation of his diplomatic career in the Bay of Pigs invasion. After hearing rumors that "a lot of refugees wanted to go back and overthrow Castro", Stevenson voiced his skepticism about an invasion, but "he was kept on the fringes of the operation, receiving...nine days before the invasion, only an unduly vague briefing by Arthur M. Schlesinger Jr." and the CIA. Senior CIA official Tracy Barnes told Stevenson and his staff that "there was going to be a clandestine operation in Cuba...it was strictly a Cuban affair. It would have some American cooperation, but only with the training and financing." According to Historian Peter Wyden, Barnes did not tell Stevenson that there would be a large-scale invasion of Cuba, nor did he provide details about the full extent of American support for, and involvement with, the Cuban rebels, nor did he tell Stevenson about the planned air strikes to destroy Castro's air force. Assistant Secretary of State Harlan Cleveland, who attended the briefing, felt that Barnes was too evasive in his description of the operation, and that it was clear that Stevenson was not to be given the full details of the invasion plan. Historian Garry Wills has written that "news of the invasion was leaking out...Castro knew the landings would occur; only Adlai Stevenson was kept in the dark" about the invasion by President Kennedy and his aides.

In John Frankenheimer's 1962 cold war thriller The Manchurian Candidate, the conniving Mrs. John Iselin (played by Angela Lansbury) makes a reference to Stevenson in a conversation with her son (played by Laurence Harvey): "Mr. Stevenson makes jokes. I do not."

During his time as UN Ambassador, Stevenson often traveled around the country promoting the United Nations in speeches and seminars. On these trips, he frequently faced opposition and protests from groups skeptical of the United Nations, such as the right-wing John Birch Society. In October 1963 Stevenson spoke in Dallas, Texas, where he was shouted down by unruly protestors led by retired General Edwin Walker's "National Indignation Convention". At one point a woman hit Stevenson on the head with a sign, leading Stevenson to remark "is she animal or human?", and telling a policeman "I don't want her to go to jail, I want her to go to school." Afterwards, Stevenson warned President Kennedy's advisers about the "ugly and frightening" mood he had found in Dallas, but Kennedy went ahead with his planned visit to Dallas in late November 1963.

After President Kennedy was assassinated, Stevenson continued to serve in his position as Ambassador to the UN under President Lyndon Johnson. As the country moved toward the 1964 presidential election, the war in Vietnam became an important campaign issue. The Republican presidential candidate, Arizona Senator Barry Goldwater, advocated victory in Vietnam—a rollback strategy that Johnson denounced as tantamount to nuclear war. Stevenson was not a major player on the Vietnam issue. He did support Johnson publicly and in private because he believed in the containment of communism, but he also wanted to start negotiations with North Vietnam through the United Nations, which Johnson rejected.

That night in her diary, she wrote, "Adlai is dead. We were together." Following memorial services at the United Nations General Assembly Hall (on July 19, 1965), and in Washington, D.C.; Springfield, Illinois; and Bloomington, Illinois, Stevenson was interred in the family plot in Evergreen Cemetery, Bloomington, Illinois. The funeral in Bloomington's Unitarian Church was attended by many national figures, including President Lyndon B. Johnson, Vice President Hubert Humphrey, and Chief Justice Earl Warren.

Stevenson has been referenced in television episodes of The Simpsons (in the episodes "Lisa the Iconoclast" and "The Secret War of Lisa Simpson" (appearing in the latter as a filmstrip, with Harry Shearer providing the cartoon Stevenson's voice), The Golden Girls, Happy Days (in the January 28, 1975, episode "The Not Making of the President") and Mystery Science Theater 3000's presentation of Manos: The Hands of Fate (a Stevenson lookalike buys a car and one of the MST3K characters comments on it). Murphy Brown briefly names her newborn son 'Adlai Stevenson'.

Stevenson comes close to being assassinated by a 12-year-old in James Patrick Kelly's Hugo Award-winning novelette 10 to 1 (1999).

Stevenson has also been referenced in films. Peter Sellers claimed that his portrayal of President Merkin Muffley in Dr. Strangelove was modeled on Stevenson. Stevenson's "Don't wait for the translation" speech to Russian ambassador Valerian Zorin during the Cuban Missile Crisis inspired dialogue in a courtroom scene in Star Trek VI: The Undiscovered Country. The historical speech itself is depicted in the 2000 film Thirteen Days with Michael Fairman playing Stevenson, as well as partially depicted in the 1974 television play The Missiles of October by Ralph Bellamy. Stevenson is also referenced in Wayne's World 2 ("Waynestock" is held in an Aurora, Illinois, park named for Stevenson), Plain Clothes (the high school is named for Stevenson), Annie Hall (Woody Allen's character tells a standup joke about the Stevenson-Eisenhower campaign) and Breakfast at Tiffany's.

The alternate history novella "Southern Strategy" by Michael F. Flynn (Alternate Generals, volume two, Baen, 2002), is told entirely from Stevenson's point of view. In a world where the Kaiser's Germany is the leader of something resembling a free world in 1956, Stevenson is a former Senator of the United States, which is in ruins after a Second American Civil War. The novella follows Stevenson's increasingly futile efforts to negotiate an armistice between League of Nations peacekeepers led by General Erwin Rommel and several disparate guerrilla-terrorist bands with differing agendas. One of the terrorist bands is led by Richard Nixon, and another is led by John Calvin King, an apparently fictional analog of Martin Luther King Jr.

Stevenson did not use television as effectively as his Republican opponent, war hero Dwight D. Eisenhower, and was unable to rally the New Deal voting coalition for one last hurrah. On election day, Eisenhower won the national popular vote by 55% to 45%. Stevenson lost heavily outside the Solid South; he carried only nine states and lost the Electoral College vote 442 to 89. In his concession speech on election night, Stevenson said: "Someone asked me...how I felt, and I was reminded of a story that a fellow townsman of ours used to tell – Abraham Lincoln. He said he felt like the little boy who had stubbed his toe in the dark. He said that he was too old to cry, but it hurt too much to laugh."

Following this speech, the Illinois delegation (led by Jacob Arvey) announced that they would place Stevenson's name in nomination, and Stevenson called President Truman to ask if "he would be embarrassed" if Stevenson formally announced his candidacy for the nomination. Truman told Stevenson "I have been trying since January to get you to say that. Why should it embarrass me?" Kefauver led on the first ballot, but was well below the vote total he needed to win. Stevenson gradually gained strength until he was nominated on the third ballot. Historian John Frederick Martin says party Leaders selected him because he was "more moderate on civil rights than Estes Kefauver, yet nonetheless acceptable to labor and urban machines—so a coalition of southern, urban, and labor Leaders fell in behind his candidacy in Chicago." Stevenson's 1952 running mate was Senator John Sparkman of Alabama.

In the 2016 Movie, Bogie and Bacall, Stevenson is portrayed by actor Ryan Paevey

Adlai Stevenson II was inducted as a Laureate of The Lincoln Academy of Illinois and awarded the Order of Lincoln (the State’s highest honor) by the Governor of Illinois in 1965 in the area of Government.