Born in Albuquerque, New Mexico, French Stewart studied at the American Academy of Dramatic Arts and toured in regional theatre for seven years before breaking into television with the role of Razor Dee, a spaced-out DJ on the final season of The New WKRP in Cincinnati in 1992.

Stewart earned his Screen Actors Guild card while working for Hanna-Barbera's Shakey Quakey tour, having been later terminated for removing the head of his costume in front of children. In 1996, he was cast on 3rd Rock from the Sun, which lasted for six seasons; on the show, Stewart was noted for his talents at physical comedy and his characteristic "squinting" facial expression, caused by a genetic defect that affects his abducens nerve.

Major motion picture credits for French Stewart incude his film debut on Stargate (1994) as Lieutenant Ferretti, followed by Leaving Las Vegas (1995), McHale's Navy (1997), Love Stinks (1999). Animated credits include the voice of Bob on the short-lived animated series God, the Devil and Bob (2000), and Disney's animated Hercules (1998) as Icarus. Since the end of 3rd Rock in 2001, Stewart has appeared in a number of minor roles, mainly in situation comedies (such as Just Shoot Me!, The Drew Carey Show, and That 70's Show, from the same creators of 3rd Rock from the Sun) and comedy films, although he did have major roles in the direct-to-video films Home Alone 4 (2002) and Inspector Gadget 2 (2003).

Stewart has appeared as a guest on MADtv once, in the intro, humourously and profusely exclaiming that he is not Harry from 3rd Rock from the Sun. He also appeared in a cameo on Seinfeld.

He has been portrayed on Saturday Night Live's parody of Celebrity Jeopardy!, played by actor Jimmy Fallon (who answered the final Jeopardy question, write a number, with Threeve. He wagered "$Texas" (Texas with a dollar sign).

Stewart married actress Katherine La Nasa on May 19, 1998. They met when she had a guest appearance on a 1996 episode of 3rd Rock from the Sun.