Joseph Sill Clark Jr. (October 21, 1901 – January 12, 1990) was an American writer, lawyer and politician. A member of the Democratic Party, he served as the 90th 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. Clark earned a reputation as a strong supporter of civil rights and worked to appoint liberal committee members from his perch on the Democratic Steering Committee. Clark narrowly won re-election in 1962 but was defeated in 1968 by Congressman Richard Schweiker. His defeat is generally credited to his support of gun control and opposition to the Vietnam War. After leaving office, Clark became a professor at Temple University.

One of two children, Joseph Clark was born in Philadelphia to Joseph Sill Clark Sr. and Kate Richardson Avery. His father, a longtime lawyer in the Germantown section of the city, was also a national tennis champion who won the 1885 U.S. National Championship in doubles with Dick Sears. His mother, whose family owned Avery Island in Louisiana, was the niece of Edmund McIlhenny, the inventor of Tabasco sauce. Clark was raised in the Chestnut Hill section of Philadelphia, and received his early education at Chestnut Hill Academy. He then attended Middlesex School in Concord, Massachusetts, where he played on the school's baseball and football teams. He graduated from Middlesex in 1919 as class valedictorian.

Clark studied at Harvard University, where he was a member of the baseball and track teams. He won several prizes, including the John Harvard scholarship for high academic distinction. He was elected to Phi Beta Kappa and graduated magna cum laude in 1923 with a Bachelor of Science degree in government, history and economics. Clark, who had spent time at the Bar B C Dude Ranch in Jackson Hole, Wyoming, became a partner in the nearby Double Diamond Dude Ranch in 1924. He later returned to Philadelphia and enrolled at the University of Pennsylvania Law School, where he was a member of St. Anthony Hall and editor of the Law Review. He earned his Bachelor of Laws degree in 1926.

In December 1926, Clark was admitted to the bar and became an associate with his father's law firm of Clark, Clark, McCarthy and Wagner. That same year, he made his first entry into politics when he unsuccessfully ran as a progressive candidate for Republican committeeman. In 1928, he left the Republican Party and became a Democrat, supporting Al Smith in that year's presidential election.

During the 1928 election, Clark founded the Democratic Warriors Club with Richardson Dilworth, beginning a long political partnership between the two. Both men became active in the reform movement to end corruption in city government, which was long controlled by a Republican machine. Clark was an unsuccessful candidate for the Philadelphia City Council in 1933, with Dilworth serving as his campaign manager. The following year, he joined the firm of Dechert, Bok, Smith and Clark, and continued to practice law for seven years. He managed Dilworth's unsuccessful campaign for the Pennsylvania State Senate in 1934. From 1934 to 1935, he served as Deputy Attorney General of Pennsylvania. In this capacity, he engaged in trial work related to the closing of banks.

In August 1941, Clark enlisted in the United States Army Air Forces (AAF) and was assigned as a captain in the Officers' Reserve Corps Headquarters at Washington, D.C. He later became director of the Organizational Planning Headquarters with the AAF. Following the entry of the United States into World War II, he was transferred to the China Burma India Theater as deputy chief of staff to General George E. Stratemeyer. He briefly served as acting chief of staff to General Stratemeyer, and attained the rank of colonel on October 15, 1943. He was awarded the Bronze Star Medal, Legion of Merit, and Military Order of the British Empire. Returning to the United States in September 1945, he accompanied General Stratemeyer to Washington, D.C., where he helped design plans to defend the nation against air raids.