Answer: Joseph Sill Clark, Jr.

Elected in 1951, Joseph Sill Clark, Jr. served as mayor of Philadelphia for four years, the city’s first Democrat to hold the office in 67 years. A Democratic reform politician, Clark later served as a U.S. senator from Pennsylvania.

Clark was born in Philadelphia in 1901 to tennis star Joseph Sill Clark and Kate Avery.  He graduated magna cum laude from Harvard University in 1923 and received a law degree from the University of Pennsylvania in 1926. His initial career as a lawyer was interrupted when he was drafted during World War II.  After the war, Clark became involved in local politics and made a name for himself as a reform Democrat. 

Clark served as campaign manager for political ally and personal friend Richardson Dilworth's unsuccessful mayoral run in 1947.  Political success came two years later when Dilworth was elected city treasurer, Clark was elected city controller, and Democrats won fifteen of the seventeen council seats.  Clark ran for mayor himself in 1951 and was successfully elected, becoming the city’s first Democrat in 67 years to hold the office.  In 1956, rather than seek reelection as mayor, Clark instead ran for the U. S. Senate. He won the office and was reelected to the position in 1962.  Representing Pennsylvania, Clark denounced municipal corruption, supported civil rights, fought segregation in private schools, rallied for public housing, and initiated an era of urban renewal and development.

HSP’s holds an extensive collection of Joseph Sill Clark papers (#1958), which mostly covers Clark's years in the U. S. Senate, but a portion is devoted to his career in Philadelphia law and politics.  The collection contains material gathered by his staff, such as reports, memoranda, clippings, news releases, articles, correspondence, all on issues and events with which Clark was involved.
 

Image: "Partners in Pennsylvania's Progress," October 21, 1962, a paid advertisement for Joseph Clark and Mayor Richardson Dilworth, Richardson Dilworth papers (Collection 3112), The Historical Society of Pennsylvania.