While attending law school, Stennis won a seat in the Mississippi House of Representatives, holding office from 1928 to 1932. After serving as a prosecutor and state judge, Stennis won a special election to fill the Senate vacancy that arose following the death of Theodore G. Bilbo. He won election to a full term in 1952 and remained in the Senate until he declined to seek re-election in 1988. Stennis became the first Chairman of the Senate Ethics Committee and also chaired the Committee on Armed Services and the Committee on Appropriations. He also served as President pro tempore of the Senate from 1987 to 1989. In 1973, President Richard Nixon proposed the Stennis Compromise, whereby the hard-of-hearing Stennis would be allowed to listen to, and summarize, the Watergate tapes, but this idea was rejected by Special Prosecutor Archibald Cox.