At the age of fifteen, she auditioned for the role of Kelly Kapowski on the television show Saved by the Bell, which was highly publicized and received a great deal of fanfare from the same magazine that discovered her. In the end, Thiessen's publicity from that magazine had landed her the role. She was nominated for, but did not win, the Young Artist Award for Outstanding Young Ensemble Cast. She was also nominated in 1989, 1991, and 1992. She continued to play the role of Kelly in the short-lived spin-off Saved by the Bell: The College Years. To broaden her visibility on television, Thiessen began making television appearances with guest starring roles on several television shows and television movies, as well as in film. Including Saved by the Bell: Hawaiian Style (1992), A Killer Among Friends (1992), Son in Law (1993), and Saved by the Bell: Wedding in Las Vegas (1994). During her years while on Saved by the Bell, Thiessen was seen in guest roles on Charles in Charge (1990), Married... with Children (1990), The Hogan Family (1990), Step by Step (1992), and The Powers That Be (1992).

After her tenure on Saved by the Bell ended in 1994, producer Aaron Spelling then cast Thiessen in the hit teen-drama Beverly Hills, 90210 playing the bad girl vixen Valerie Malone (1994–1998). Thiessen then began to take on dramatic roles in several made-for-television movies, including The Stranger Beside Me (1995), She Fought Alone (1995), Sweet Dreams (1996), and Buried Secrets (1996). Thiessen later guest-starred on NewsRadio (1999) and the sitcom Two Guys and a Girl and starred in the short-lived action drama series Fastlane. She also guest-starred on Just Shoot Me! (2001) playing Amy Watson and the short-lived Good Morning, Miami (2003–2004) playing Victoria Hill.

Her transition into film has not gone well thus far, as most of her movies have been universally panned by film critics. In 2002, she appeared in the film Hollywood Ending, directed by Woody Allen.
In 2005, Thiessen directed a 21-minute short film entitled, Just Pray. It placed at several film festivals and was the first major effort from her latest production company, Tit 4 Tat Productions.

In January 2007, Thiessen joined the cast of ABC's drama What About Brian, playing the boss of Brian, and a potential love interest for Adam. In May 2007, ABC canceled the series.
On December 2, 2008, TV Guide reported that Thiessen was cast in White Collar, a new USA Network television pilot.

Thiessen appeared as a celebrity client for Lonni Paul, one of the final three contestants on the season 4 HGTV interior designing reality series Design Star, on the September 6, 2009 episode. Interior designer Lonni Paul was given the task of redesigning Thiessen's guest bedroom with an eco-friendly theme. Her husband Brady Smith also appears in this episode.

Filmography:

1989 Saved by the Bell
1990 Married... with Children
1990 Charles in Charge
1992 Step by Step
1992 Blossom
1992 Saved by the Bell: Hawaiian Style
1992 The Powers That Be
1992 A Killer Among Friends
1993 Son in Law
1993 Saved by the Bell: The College Years
1994 Saved by the Bell: Wedding in Las Vegas
1994 Beverly Hills, 90210
1995 The Stranger Beside Me
1995 She Fought Alone
1996 Sweet Dreams
1996 Buried Secrets
1999 Speedway Junky
1999 From Dusk Till Dawn 2: Texas Blood Money
1999 Love Stinks
1999 NewsRadio
2000 Ivans XTC
2000 The Ladies Man
2000 Shriek If You Know What I Did Last Friday the Thirteenth
2001 Everything But the Girl
2001 A Christmas Adventure from a Book Called Wisely's Tales
2001 Just Shoot Me!
2002 Hollywood Ending
2002 Fastlane
2005 Just Pray
2006 Stroller Wars
2007 Pandemic
2007 What About Brian
2008 Cyborg Soldier
2009 White Collar