Joseph Sill Clark (October 21, 1901 - January 12, 1990) was an U.S. lawyer and Democratic Party politician in the mid-20th century. He served as the mayor of Philadelphia from 1952 until 1956, and as a United States Senator from Pennsylvania from 1957 until 1969.
Clark was a native of Philadelphia. After graduating from Middlesex School, he received his bachelor's degree magna cum laude from Harvard University in 1923 and his law degree from the University of Pennsylvania in 1926. He was a fellow of the American Academy of Arts and Sciences and a member of the American Philosophical Society.
Clark began his career as a lawyer when, in 1941, he was called up to military service for World War II, becoming a colonel in the United States Army Air Forces before mustering out in 1945. He won the Bronze Star, the Legion of Merit, and the OBE for his wartime service in the China-Burma-India theater.
After the war, Clark became involved in Philadelphia city politics, first winning the city controller position in 1949, then moving up to mayor three years later. On the strength of his mayoral record, he won the Democratic nomination for the U.S. Senate in 1956, and went on to beat incumbent James H. Duff in the general election.
After losing the 1968 senatorial election to Republican Richard Schweiker, Clark retired from politics, and lived the rest of his life in Philadelphia.