Adlai Stevenson III was born in Chicago. He attended the Milton Academy in Massachusetts, Harrow School in England, and Harvard College.
He received a law degree in 1957 from Harvard Law School.
Stevenson was commissioned as a lieutenant in the U.S. Marine Corps in 1952, served in Korea and was discharged from active duty in 1954. He continued to serve in the Marine Reserves and was discharged in 1961 as a captain.
In 1957, Stevenson went to work as a clerk for a Justice of the Illinois Supreme Court and worked there until 1958 when he joined the law firm of Brown and Platt.
In 1964, Stevenson was elected to the Illinois House of Representatives as an at-large representative due to reapportionment problems, serving from 1965 to 1967. He then served as Illinois Treasurer (1967–1970).
After U.S. Senator Everett Dirksen (R-Ill.) died in office in 1969 and Ralph Tyler Smith was appointed to the seat, Stevenson defeated Smith in a 1970 special election by a 58% to 42% margin to fill Dirksen's unexpired term. Stevenson introduced legislation requiring an end to all foreign aid to South Vietnam by June 30, 1975. He authored the International Banking Act, the Stevenson Wydler-Technology Innovation Act and its companion, the Bayh Dole Act, to foster cooperative research, organize national laboratories for technology utilization and commercialization, permit private sector interests in government-funded research. He was the first Chairman of the Senate Ethics Committee charged with implementing a code of ethics he helped draft. Stevenson was also chairman of a Special Senate Committee that reorganized the Senate and served on the Democratic Policy Committee. Inter alia, he also conducted the first in-depth congressional study of terrorism as Chairman of the Subcommittee on the Collection and Production of Intelligence, leading to introduction of the Comprehensive Counter Terrorism Act of 1971 with warnings of "spectacular acts of disruption and destruction" and an amendment that proposed reducing assistance for Israel by $200 million until the president could certify that settlements polices of the newly elected Likud Government of Israel were consistent with U.S. policy. His amendment received seven votes.
Stevenson was re-elected to the seat in 1974, and in 1980 declined to stand for re-election, thus serving in the U.S. Senate from 1970 to 1981.
Stevenson was encouraged to run for president in 1976 by Mayor Richard J. Daley of Chicago. He declined to run, and was one of the finalists for vice president at the Democratic Convention that year. Senator Walter Mondale of Minnesota was nominated for vice president.
Stevenson ran for Governor of Illinois in 1982 and 1986, losing both elections to James R. Thompson. In 1982, the initial vote count showed Stevenson winning; however, the final official count showed him losing by 0.14 percent. Stevenson promptly petitioned the Illinois Supreme Court for a recount and presented evidence of widespread election irregularities, including evidence of a failed punch card system for tabulation of votes. Three days before the gubernatorial inauguration, the court denied the recount by a one-vote margin, asserting that the Illinois recount statute was unconstitutional.
In the 1986 statewide Democratic primaries, Democratic voters nominated allies of Lyndon LaRouche for Lieutenant Governor and Secretary of State. Stevenson objected to their platform and refused to appear on the same ticket. Instead, he organized the Solidarity Party to provide an alternate slate for Governor, Lieutenant Governor and Secretary of State, which was endorsed by the regular Democratic organization. Persuading Democrats to vote for most of the Democratic ticket as well as the Solidarity candidates for Governor, Lt. Governor and Secretary of State was an unconventional strategy; however, Stevenson and the candidate for Lieutenant Governor position, Mike Howlett, won 40% of the vote.
Since leaving the senate, Stevenson has been active in business and cultural relations with east Asia. He is chairman of SC&M Investment Management Corporation, and co-chairman of HuaMei Capital Company (the first Chinese-American investment bank).
He has also held many positions with non-profit organizations in this area. He has served as chairman of the Japan-America Society of Chicago, the Midwest U.S.-Japan Association, and the Midwest U.S.-China Association, and as president of the U.S. Committee of the Pacific Economic Cooperation Council (PECC). He is also co-chairman of the PECC's Financial Market Development Project, a member of the U.S.-Korea Wisemen Council, and sits on the Board of Directors of the Korea Economic Institute.
Stevenson has been honored by the government of Japan with the Order of the Sacred Treasure with gold and silver star and is an Honorary Professor of Renmin University in China.
He is chairman of the international Adlai Stevenson Center on Democracy housed at the family home, a national historic landmark, near Libertyville, Illinois. The Center brings practitioners from the real world of politics together with scholars and experts from many parts of the world to address systemic challenges to democratic systems of government and other subjects of public concern. Stevenson is author of The Black Book—which records American history and culture from within its politics as his family knew it over five generations, starting with his great great grandfather, Jesse Fell, who was Abraham Lincoln's patron and persuaded him to run for President. Stevenson contrasts what he sees as the values that created America with those that now undermine it. The Black Book has been published by the China Academy of Social Studies Press as American History and the Story of the Black Book.
Stevenson is a member of the ReFormers Caucus of Issue One.
On 8 December 2012 Stevenson endorsed the proposal for the United Nations Parliamentary Assembly (UNPA), one of only six persons who served in the United States Congress ever to do so.
Stevenson's great-grandfather Adlai E. Stevenson I was Vice President of the United States (1893–1897) during Cleveland's two terms. His grandfather Lewis Stevenson was Illinois secretary of state (1914–1917). His father, Adlai Stevenson II, was governor of Illinois, Ambassador to the United Nations, and two-time Democratic presidential nominee. Actor McLean Stevenson was his third cousin.
Adlai Stevenson IV, Stevenson III's son, became a television reporter in Chicago in the 1980s. It is reported that when asked if he liked his name, he said he intended to become "Adlai the Last". However, in the summer of 1994, Adlai Ewing Stevenson V was born.
Adlai Stevenson III 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 1981 in the area of Government. He is the recipient of numerous honorary degrees and other awards, including Order of the Sacred Treasure with Gold and Silver Star from Japan and Honorary Professor, Renmin University, Beijing.