Justin's father is from an Ashkenazi Jewish family that had been in the United States since the mid 1800s on several branches, and was originally from France and Germany. Among Justin's paternal ancestors is Isaias W. Hellman, a founding father of the University of Southern California, and Sidney M. Ehrman, who founded the law firm Heller Ehrman. Justin's mother is of mostly British Isles descent, and is the daughter of Edwin F. Thayer, who was publisher of Industrial Marketing and Tide magazines.