Former child model and pageant winner turned successful TV actor, usually in roles of the prom queen vein. Thiessen began professionally posing at age seven, appearing in both print and TV ads as well as catalogues. After winning the title Miss Junior America in 1987, she moved to series TV, playing the squeaky clean girlfriend of the conniving Zach on the wildly popular Saturday morning teen hit, "Saved By The Bell" (NBC, 1988-93). The success of that series spawned several "Saved" TV-movies and a short-lived primetime spin-off, "Saved By The Bell: The College Years" (NBC, 1993-94). After making her feature debut in the Pauly Shore vehicle "Son-in-Law" (1993), Thiessen switched from playing wholesome to raising hell as troubled mini-diva Valerie Malone on Fox's "Beverly Hills, 90210". Producers hoped that Thiessen's character would do for "Beverly Hills" what Heather Locklear did for "Melrose Place", that is, add a bracing dose of bitchiness to spice things up.
While Thiessen's addition to "Beverly Hills 90210" did give it a much needed shot of new life, the show was past its prime and quickly becoming a soap opera rather than a drama. Thiessen, with her stylish new short hair and prominent breast implants, played up the part of the sexy bad girl. Thiessen maintained a level of believable acting, even as the storylines around her became more and more far-fetched. She felt, however, that she was increasingly being used as a sex object on the show, and in 1998, decided to leave and pursue a film career.
Thiessen appeared in two small budget films in 1999, the comedy "Love Stinks" with Bill Bellamy and Bridgette Wilson and the drama "Speedway Junky." She next appeared in the SNL spinoff film "The Ladies Man" (2000) and starred in the straight to video spoof "Shriek If You Know What I Did Last Friday the Thirteenth" (2000). Although Thiessen had made the difficult transition from child actor to successful adult star, she was finding the notoriously difficult switch from TV to film even more challenging. In 2001, she returned to television with recurring spots on "Two Guys, a Girl and a Pizza Place" and "Just Shoot Me." However, 2002 gave Thiessen her biggest film role yet, in Woody Allen's "Hollywood Ending." 2002 also saw her return as a cast member on primetime, reuniting with Bill Bellamy in the Fox Series "Fast Lane."