John William Ferrell was born in Irvine, California, to Betty Kay (Overman), a teacher, and Roy Lee Ferrell, Jr., a musician. His parents were originally from Roanoke Rapids, North Carolina.
A graduate of the University of Southern California, Ferrell became interested in performing while a student at University High School in Irvine, where he made his school's daily morning announcements over the public address system in disguised voices. He started as a member of the Los Angeles comedy/improvisation group The Groundlings, where fellow cast members Ana Gasteyer, Maya Rudolph and former Saturday Night Live (1975) repertory players such as Laraine Newman, Jon Lovitz and Phil Hartman began their careers. It was there he met Chris Kattan and the two became good friends and both went on to Saturday Night Live (1975) later. He has also appeared on several television programs, including Strangers with Candy (1999), Grace Under Fire (1993) and Living Single (1993) during his time at The Groundlings. Will also lent his voice to the armless and legless dad of cartoon family "The Oblongs".