Marshall has a theatre background and trained for the stage. When she was teaching fifth grade, the mother of one of her students suggested she enroll in a voice-over class taught by the late Daws Butler, voice of Yogi Bear and Quick Draw McGraw.
She is often cast in the roles of young male characters. Her roles have included parts in not only in American animated television series and several animated feature films, but also in Japanese anime.
Her most notable roles in American cartoons are Sheila Broflovski (1999–present, after original voice actress Mary Kay Bergman committed suicide) and Linda Stotch on the popular television show South Park as well as the title character in Doraemon and Kite, the main protagonist of the 2002 video game series .hack for PlayStation 2. Marshall voiced the talking bear Koby the Study Buddy and she also provided the English voice for the title character in the El Chavo: The Animated Series.
Outside of voice acting, Marshall has coached others on the craft, and has worked on a singing career with solo songs she would perform on stage.