Hardin started her acting career as the young star of the television series Thunder (1977–1978), and has appeared in over 70 movies and television programs since, including two 1981 Little House on the Prairie episodes (as Belinda Stevens in "The Reincarnation of Nellie", Parts 1 and 2) as well as playing Michele Pierson in the 1983 television movie Little House: Look Back to Yesterday. She starred as Baby in the short-lived 1988 television series Dirty Dancing (based on the 1987 film of the same name) and two 1992 episodes of Quantum Leap (as Abigail in "Trilogy," Parts 2 and 3). She co-starred in the 1990 dancing movie Lambada as Sandy. She appeared in Absolute Power (1997) as Christy Sullivan and played Ross's dirty talking love interest in season 1 episode 15 of Friends. She was also originally cast in Back to the Future as Jennifer Parker, but was re-cast after Eric Stoltz (the original Marty McFly) was let go. Hardin was deemed too tall to star alongside Michael J. Fox.
Hardin starred in the 2000-2001 series Cover Me: Based on the True Life of an FBI Family, and 2004-2009 she played the recurring role of Trudy Monk, the title character's deceased wife, on the USA Network series Monk. She appeared on the series NCIS as former Petty Officer Erin Toner in the episode "The Curse". In 2005, Hardin portrayed Linda Evans in Dynasty: The Making of a Guilty Pleasure, a fictionalized television movie based on the creation and behind the scenes production of the 1980s prime time soap opera Dynasty. Hardin played former corporate manager Jan Levenson on The Office since the show's debut in 2005 until 2011.
Hardin later appeared in the 2006 film Thank You for Smoking, and appears in Hannah Montana: The Movie as Robby Ray's (Billy Ray Cyrus') love interest. The film premiered on April 10, 2009.
She played the role of Fantine in the Hollywood Bowl's concert of Les Misérables in Summer 2008.
Hardin also appeared in several episodes of the popular web show, Elevator, on YouTube.
Hardin made her Broadway debut as Roxie Hart in the revival of Chicago: The Musical on December 29, 2008. She stayed with the show until February 12, 2009.
Hardin was cast as a major character in the NBC drama Outlaw, playing a powerful senior partner of an elite law firm, and the main love interest to the series' protagonist, played by Jimmy Smits.