London was born in San Diego, the son of Debbie London (née Osborn), a waitress, and Frank London, a sheet metal worker. He was raised near Tuttle, Oklahoma, and in DeSoto, Texas. His identical twin brother Jeremy is also an actor. In 2003, the two acted together in an episode of 7th Heaven titled "Smoking". Jeremy was Jason's stunt double in The Man in the Moon (1991).