She was born in Long Beach, California, and her father, Frank, was a park designer and her mother, Robyn, was a homemaker. She is of German, Greek, Turkish and Welsh descent. She told InStyle, "I'm a mutt. I have so much of everything in me, and half of it I don't even know. German on one side; Greek, Turkish and Welsh on the other. My mom is very olive-skinned; I get my blue eyes from my dad." Her brother, Todd, was a really big inspiration to her, and her biggest role models were her mother and grandmother.

As a child, she competed in many beauty pageants, and she won the Miss Junior America pageant in 1987. Her first big break in show business was her role on the television program Saved By The Bell, as nice girl Kelly Kapowski, a role she continued playing in the short lived Saved by the Bell: The College Years. This was very different from her next major role, as a duplicitous vixen, Valerie, on Beverly Hills 90210. She later starred in the sitcom Two Guys and a Girl and the short-lived action show Fastlane, as well as guest-starring on Just Shoot Me!. Her transition to film has not gone well so far, as most of her movies, including Love Stinks, have been critically panned. In 2002, she appeared in the film Hollywood Ending by Woody Allen. In 2005, she released a short (21 minute) film she had directed called Just Pray. It placed at several film festivals and was the first major effort from her new production company, Tit 4 Tat Productions.

In March of 1999, her then-boyfriend, actor David Strickland, committed suicide and from October 2001 to April 2003, she was engaged to actor Richard Ruccolo. On July 9, 2005, she married actor Brady Smith in the gardens of a private estate in Montecito, California.

Thiessen is a part of the Make-a-Wish Foundation. In February 2006, she accepted an invitation to serve on the National Advisory Council for the organization.