Moving with his family in 1906 to Bloomington, Ill., he followed in the footsteps of his grandfather, Vice Pres. Adlai E. Stevenson (1893–97), by entering the practice of law (Chicago, 1926) and devoting himself to public service. He headed the Civil Rights Committee of the Chicago Bar Association, and—in a period when isolationism was rampant in the Midwest—was chairman of the Chicago chapter of the Committee to Defend America by Aiding the Allies. From 1941 to 1944 he was special assistant to the U.S. secretary of the navy, and in 1943 he headed a Foreign Economic Administration mission to Italy to develop a U.S. relief program. Two years later he became assistant to the secretary of state and served as an adviser to the U.S. delegation to the San Francisco Conference that founded the UN. He was named senior adviser to the U.S. delegation at the first meeting of the UN General Assembly in London (1946) and was a U.S. delegate at Assembly meetings in New York (1946–47).

In 1948 Stevenson was elected governor of Illinois by a larger majority than any other candidate had received in the history of the state. His administration was characterized by far-reaching reforms: establishment of a merit system for state police, improved care and treatment of patients in state mental hospitals, greater state aid for schools, and a revitalized civil service.

In spite of his refusal to seek the presidential nomination in 1952, he was drafted by the Democratic National Convention in Chicago. He waged a vigorous campaign, but the popular appeal of wartime hero Gen. Dwight D. Eisenhower (Eisenhower, Dwight D.) proved irresistible. Stevenson was defeated a second time four years later, again by Eisenhower.

With the election of Pres. John F. Kennedy in 1960, Stevenson was appointed chief U.S. representative to the UN, holding Cabinet rank and the title of ambassador. He served until his death, helping to assuage some of the worst international tensions—brought on by the financial difficulties of the parent organization, by the Cold War, and by the sensitivity of emerging African and Asian nations to traditional Western leadership. Stevenson's published works include Call to Greatness (1954); What I Think (1956); Friends and Enemies (1958); Looking Outward: Years of Crisis at the United Nations (1963); and The Papers of Adlai E. Stevenson (1972– ).

His eldest son, Adlai E. Stevenson III, was elected to the U.S. Senate from Illinois in 1970 and again in 1974 (retiring in 1981), after serving in the state legislature (1965–67) and as state treasurer (1967–70).