William Scranton was born on July 19, 1917 while the Scranton family was on vacation at a cottage in Madison, Connecticut. He was the son of Worthington Scranton, a wealthy Pennsylvania businessman, and Marion Margery (Warren) Scranton, a member of the Republican National Committee for over two decades. Despite her own involvement in politics, his mother tried to dissuade him from entering politics because of his childhood struggles with asthma. She feared that the stress of campaigning would be detrimental to his frail health. She died just before her son's election to Congress in 1960.
Scranton family members, George W. Scranton and his brother Selden, were the founders and patriarchs of the city of Scranton, Pennsylvania. He was the grandnephew of Joseph A. Scranton, a Representative from Pennsylvania. He was also a nephew by marriage of former U.S. Supreme Court Justice David Davis, a confidante of President Abraham Lincoln. His maternal ancestors came to America on the Mayflower.
Scranton's father was the son of William Walker Scranton (1844-1916) and Katharine Maria Smith (1851-1935). His father was an industrialist, general manager of the Lackawanna Iron & Coal Company founded by George W. Scranton, among other interests in Scranton. Katherine Smith was the daughter of Worthington Curtis Smith, who served as a Congressman from Vermont. Katherine M. Smith's uncle was J. Gregory Smith, who served as Governor of Vermont. Her cousins included Edward Curtis Smith, who also served as governor. Her grandfather, John Smith, also served as a Congressman from Vermont. Scranton's genealogical line runs from John Smith (great-great-grandfather) to Worthington C. Smith (great-grandfather) to Katherine Maria Smith Scranton (paternal grandmother) to Worthington Scranton (father) to Scranton.