Michael Joseph Mansfield was an American politician and diplomat. A member of the Democratic Party, he served as a U.S. Representative (1943–1953) and a U.S. Senator (1953–1977) from Montana. He was the longest-serving Senate Majority Leader, serving from 1961 to 1977. During his tenure, he shepherded Great Society programs through the Senate, but strongly opposed the Vietnam War.
After retiring from the Senate, Mansfield served as U.S. Ambassador to Japan from 1977 to 1988, and upon retiring as ambassador, was awarded the nation's highest civilian honor, the Presidential Medal of Freedom (1989), in part for his role in the impeachment of Republican President Richard Nixon. Mansfield is the longest serving American ambassador to Japan in history.
After his ambassadorship, Mansfield served for a time as a senior adviser on East Asian affairs to Goldman Sachs, the Wall Street investment banking firm.