Her acting breakthrough came with the role of Ryan McBride on Baywatch Nights (1995–1997), and she earned four Screen Actors Guild Award nominations for playing Abbie Carmichael on Law & Order (1998–2001).

Harmon also appeared in a number of film roles, which notably include voicing Barbara Gordon in the animated Batman Beyond: Return of the Joker (2000) and Ronica Miles in Agent Cody Banks (2003).

She has starred in a number of Lifetime network's original projects, which include the biographical special Video Voyeur (2002) and the critically acclaimed feature Living Proof (2008).

She worked as a runway model for Calvin Klein, Giorgio Armani, and Donna Karan and appeared on the covers of ELLE, Cosmopolitan, and Esquire.

During this time, she also voiced Barbara Gordon in the animated film Batman Beyond: Return of the Joker, succeeding Stockard Channing in that role.

[9] In 2006, Harmon co-starred with Cuba Gooding Jr. and James Woods in the direct-to-DVD political suspense drama End Game, and appeared as Rose in the acclaimed Western film Seraphim Falls.

Harmon was one of five actresses who posed discreetly nude for the May 2008 issue of Allure magazine, alongside Gabrielle Union, Zoe McLellan, Jill Scott, and Ana Ortiz.

[17] In June 2021, Harmon had signed a multi-picture development deal with Lifetime where she hosted the documentary series Cellmate Secrets and starred as Hazel King in Buried in Barstow.