After graduation from Highland Park High School in 1990, Harmon won the Spectrum Model Search and soon was gracing the runways for such noted designers as Calvin Klein, Versace, Donna Karan, and Valentino, among others. She went on to model for Vogue, Cosmopolitan, GQ, Esquire, and Harper's Bazaar. During this time, she also began studying acting.

Harmon moved to Los Angeles in the mid-1990s to concentrate on an acting career. In 1995, she was 'discovered' when she met producer-actor David Hasselhoff on a transcontinental flight and impressed him enough to be offered the lead in the detective series spin-off Baywatch Nights. Harmon quickly segued to ABC's short-lived drama C-16 (1997–98), playing a rookie FBI agent.

Harmon's first movie gig was in the independent film Lawn Dogs (1997) with Sam Rockwell. In 1998, she was landed the role of Assistant District Attorney Abbie Carmichael on the award-winning NBC series Law & Order. She has also appeared in episodes of Law & Order: SVU.

In June 2001 Harmon married New York Giants football player Jason Sehorn. The couple is expecting their first child in October 2003.