Harmon began acting in 1995 after being discovered on a plane by David Hasselhoff. She then starred in several television series, including Baywatch Nights, Baywatch and C-16: FBI; she also appeared in the 1998 film, Lawn Dogs, which received only a limited theatrical release. In the early 2000s, Harmon became better known as playing Abbie Carmichael on the series Law & Order, a role she played from 1998 to 2001. During this time, she also voiced Barbara Gordon in the animated film, Batman Beyond: Return of the Joker, replacing Stockard Channing, as well as appearing on Law & Order.

Harmon left Law & Order to concentrate on her film career, saying that she preferred working in film than television. After appearing in the 2001 direct-to-video film Good Advice, she had a major role in Agent Cody Banks, playing the CIA handler of a teenage agent (Frankie Muniz). The film opened in March 2003 and became a minor hit, grossing $47 million. Although Harmon was originally