Born in Kipling, Mississippi, Stennis received a bachelor's degree from Mississippi State University (then Mississippi A&M) in 1923, and a law degree from the University of Virginia, where he was a member of Phi Beta Kappa, in 1928. While in law school, he won a seat in the Mississippi House of Representatives, where he served until 1932. Stennis served as a prosecutor from 1932-37, and as circuit judge from 1937-41, both for Mississippi's sixteenth judicial district.
Upon the death of Senator Theodore Bilbo in 1947, Stennis won the special election to fill the vacancy, winning the seat from a field of five candidates (including two sitting Congressmen). He would remain in the Senate until 1989.
A member of the Democratic party, Stennis was noted as the first of that party to publicly criticize Senator Joseph McCarthy on the Senate floor during the red scare. He also wrote the first code of ethics for the Senate, and was the first chair of the Senate ethics committee.
Stennis' record on civil rights was mixed throughout his long career: in the 1950s and '60s he vigorously opposed such legislation as the Voting Rights Act, but by the 1980s he had come full circle and regularly voted for bills supporting the rights of women and minorities.
In 1973 Stennis was almost fatally wounded by two gunshots after being mugged outside his Washington home. He lost his left leg to cancer in 1984.
He was unanimously selected President Pro Tempore of the Senate during the 100th Congress. During his Senate career he chaired, at various times, the Select Committee on Standards and Conduct, the Armed Services committee, and the Appropriations committee. It was due to his work with the Armed Services committee (1969-80) that he became known as the "Father of America's Modern Navy ."
Declining to run for re-election in 1988, Stennis retired from the Senate and took a teaching post at Mississippi State. He died in Jackson, Mississippi at the age of 93.
Stennis had a political career spanning 60 years, and never lost an election. His was the fourth-longest Senate career in history; his 42-year continuous tenure in the Senate was, by one year, second in longevity only to that of Strom Thurmond. The John C. Stennis Space Center and the aircraft carrier USS John C. Stennis are named in his honor.
John Stennis is buried at Pinecrest Cemetery in DeKalb County, Mississippi. He and his wife, the former Miss Coy Hines, had two children, John Hampton and Margaret Jane.