Born in Albuquerque, New Mexico, 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.
Stewart has appeared as a guest on MADtv once, in the intro, and exclaimed that he is not Harry from 3rd Rock from the Sun. He also appeared in a cameo on Seinfeld.