At the age of twelve, Stewart "fell in love" with acting when her junior high school teacher introduced her to Ashland, the location of the Oregon Shakespeare Festival. Throughout her teenage years at Truckee High School until her graduation in 1987, she participated in school plays and musicals. Stewart's dream of becoming a stage performer led her to the Pacific Conservatory of Performing Arts, an academy in Santa Monica, where she hoped to hone her acting skills.
Once there, however, she became "frustrated with the lack of theater." Grey DeLisle, a close personal friend, suggested that she try voice acting. Stewart quickly became interested in the idea, and a month after obtaining an agent, acquired her first job as a radio host, and soon afterward became the permanent voice of most every female character on South Park.