Joseph Sill Clark Jr. was an American author, lawyer and politician. A member of the Democratic Party, he served as the 116th Mayor of Philadelphia from 1952 to 1956 and as a United States Senator from Pennsylvania from 1957 to 1969. Clark was the only Unitarian Universalist elected to a major office in Pennsylvania in the modern era.
The son of attorney and tennis player Joseph Sill Clark Sr., Clark pursued a legal career in Philadelphia after graduating from the University of Pennsylvania Law School. He became involved in a reform movement that sought to break the power of the city's Republican political machine. After serving in the United States Army Air Forces during World War II, Clark won election as city controller in 1949. In this capacity, he investigated and publicized scandals in the city government. In 1951, Clark won election as Mayor of Philadelphia, becoming the first Democrat to do so since 1884. As mayor, he sought to reduce corruption in city government and created low-income housing projects.
After one term as mayor, Clark narrowly defeated incumbent Republican Senator James H. Duff in the 1956 Senate election.