Warlock was born William Alan Leming in Gardena, California, the son of Dick Warlock (né Richard Anthony Leming), a successful Hollywood stunt man who did stunts for Kurt Russell and was the stunt driver in the Disney picture, The Love Bug. His siblings include Lance Warlock and a sister, Rhonda. After graduation from Birmingham High School in Van Nuys, California in 1979, his father landed him a stunt job as Robin Williams's stunt double in a Mork & Mindy episode, one that was produced by Garry Marshall. Several months later, Billy auditioned for Marshall and won his first major television role as Leopold "Flip" Phillips, Roger's brother (and the Cunninghams' younger nephew), in the ninth season of Happy Days.