Frances Arnold was born on July 25, 1956, in Pittsburgh suburb Edgewood, Pennsylvania, United States, to the family of William Howard Arnold and Josephine Inman Routheau. She is the granddaughter of Lieutenant General William Howard Arnold. Arnold was the third of five children in a suburban family. Her mother was a homemaker. Her father, a nuclear physicist, helped Westinghouse Electric Company develop nuclear reactor plants. She greatly admired her father but rebelled against his conservative values. A family tradition of military service and the teachings of the Catholic Church lent themselves to an atmosphere of strong discipline in the Arnold home. From an early age, Frances Arnold displayed a skeptical, questioning spirit and a strong streak of independence that put her at odds with her tradition-minded parents. They quarreled over the Vietnam War, and teenage Frances hitchhiked to Washington, D.C to participate in antiwar demonstrations.
By the time Frances Arnold reached high school, this conflict became irreconcilable. Still in her teens, Frances moved out of her parents’ house, supporting herself with odd jobs and renting her own apartment in the city. By 17, she left them and her four brothers to move into her own apartment in Squirrel Hill (although still stopping by their home to do laundry).