Her roles as Kelly Kapowski on NBC's Saved by the Bell (1989–1993) and its spin-off media, and as Valerie Malone on Fox's Beverly Hills, 90210 (1994–98) established her as a teen idol of the 1990s.

As an adult, she has played Wilhemina 'Billie' Chambers in Fox's Fastlane (2002–2003), Natasha Drew in ABC's What About Brian (2007), Elizabeth Burke in USA Network's White Collar (2009–2014), and Lori Mendoza in Netflix's Alexa & Katie (2018–2020).

Thiessen has appeared in several films, such as Son in Law (1993), Speedway Junky (1999), From Dusk Till Dawn 2: Texas Blood Money (1999), Love Stinks (1999), Ivans Xtc (2000), Shriek If You Know What I Did Last Friday the 13th (2000), The Ladies Man (2000), Hollywood Ending (2002) and Cyborg Soldier (2008).

In 1989, she was named Cover Girl's "Model of the Year", and landed her first role of Kelly Kapowski on NBC's television sitcom Saved by the Bell, starring until its end in 1993.

In 1994, after her tenure on Saved by the Bell had ended, Thiessen landed the role of Valerie Malone in Fox's Aaron Spelling-produced hit teen-drama Beverly Hills, 90210, to fill the void after Shannen Doherty's departure.

While starring in 90210, Thiessen began to take on continuous dramatic leading roles in television films: The Stranger Beside Me (1995) and She Fought Alone (1995), Sweet Dreams and Buried Secrets in 1996.

After her departure from the show, she appeared in a number of films, including Speedway Junky (1999), From Dusk Till Dawn 2: Texas Blood Money (1999), Love Stinks (1999) and The Ladies Man (2000); she also guest-starred on NewsRadio (1999), Two Guys and a Girl (2000), Just Shoot Me!

[25] In November, it was announced that Thiessen would host the MTV comedy clip show Deliciousness, a spinoff of that network's previous program, Ridiculousness.

[27] Thiessen was a teen idol during the 1990s, starting from her role as cheerleader Kelly Kapowski on Saved by the Bell and then playing bad girl Valerie Malone on Beverly Hills, 90210.

In a much-discussed moment, Justin Bieber appeared wearing a T-shirt with a photo of Thiessen in her Saved by the Bell days at the 2011 MuchMusic Video Awards,[28] admitting his admiration for her.

[30][31][32][33] In early 1992, Thiessen started dating her future 90210 co-star Brian Austin Green, introduced by mutual friend David Faustino;[34] they moved in together in July 1994, then broke up the next year.