Which Supreme Court justice served the longest tenure in history?

William O. Douglas served the longest tenure of any U.S. Supreme Court justice, remaining on the Court for 36 years, 7 months, and 8 days.

Douglas joined the Court in 1939 after President Franklin D. Roosevelt nominated him to replace Justice Louis Brandeis. He served until his retirement in 1975, spanning the presidencies of Roosevelt, Truman, Eisenhower, Kennedy, Johnson, Nixon, and Ford.

Douglas was an associate justice, not chief justice. That distinction explains a common mix-up: John Marshall holds the record for longest service as chief justice, with more than 34 years in the role. Oliver Wendell Holmes Jr. also served for three decades, but his tenure was shorter than Douglas’s.

Douglas was known for his broad interpretation of constitutional liberties, environmental writing, and prolific judicial output. His record-setting tenure is measured by total time as a justice, making his name the canonical answer to this question.