Avildsen was born in Oak Park, Illinois, the son of Ivy (née Guilbert) and Clarence John Avildsen. After starting out as an assistant director on films by Arthur Penn and Otto Preminger, John Avildsen received his first success with the low budget feature Joe (1970) which received critical acclaim for star Peter Boyle and moderate box office business.
Avildsen has an estranged son named Ash (born November 5, 1981), who is well known for founding and operating Sumerian Records.
An upcoming documentary on the life, career, and films of Avildsen is currently in production. John G. Avildsen: King of the Underdogs (2014) is directed and produced by Derek Wayne Johnson and is inspired by the new book The Films of John G. Avildsen: Rocky, The Karate Kid, and Other Underdogs written by Larry Powell and Tom Garrett.