Born in Los Angeles into a political family; his grandfather Adlai E. Stevenson had been Vice President of the United States. Raised in Bloomington, Illinois and educated at the Choate School and at Princeton and Northwestern University. He got his BA at Princeton in 1922 and a law degree at Northwestern in 1926.
After university he practiced law in Chicago. He moved into federal government in 1931, working with New Deal initiatives. During the war he worked in Washington as assistant to the Secretary of the Navy. Post-war he was a delegate to the United Nations in 1946 and 1947. He was elected governor of Illinois in 1948 for the Democrats.
His popularity as governor and pressure from Truman led him to stand against Dwight D. Eisenhower in 1952. Stevenson secured only nine states and lost the Electoral College vote 442 to 89. Despite his defeat Stevenson was effective head of the Democrats and worked towards a second nomination in 1956. Standing against Eisenhower he lost again, winning only 73 Electoral votes - his concentration on foreign policy over domestic issues and Eisenhower's personal popularity were key issues. In 1960 he stood as candidate but was defeated at the Democratic National Convention by John F. Kennedy.
Following Kennedy's victory Stevenson was appointed ambassador to the UN, where he worked hard to support US policies, some of which he was personally opposed to. His most famous moment came on October 25, 1962 during the Cuban missile crisis, when he gave a presentation at an emergency session of the UN Council. He showed photographs that proved the existence of missiles in Cuba, just after Soviet ambassador Zorin had said they did not exist.
Stevenson's eldest son, Adlai Ewing Stevenson III, also went into politics as a Senator.