Tim Thomerson was born in Coronado, California. He was brought up in Hawaii and in San Diego. Following a stint in the National Guard, Thomerson chose to become an actor, while taking a job as a set builder and prop man at the Old Globe Theatre, which is famous for its Shakespearean productions. After meeting character actor Anthony Zerbe, he took Anthony's advice and studied acting, at the Herpolscheimer School of Performing Arts, with Stella Adler in New York City.
After his college tenure had ended, Thomerson began a career as a stand-up comedian, honing his skills. He appeared in clubs that included The Bitter End in Greenwich Village, Bud Friedman's Improvisation, Catch a Rising Star, and Catskills.
Thomerson's wife Teri Blythe has worked extensively in Hollywood as a script supervisor, make-up artist, casting director, and producer.
Thomerson was good friends with the late actor Brion James. Both served together as cooks in a tank company while in the Army Reserve in California. After their service, they both went to New York to break into acting. They also acted together as a pair of Cyborg hit-men in Nemesis.