Hardin was born in Houston, Texas, the daughter of acting manager Diane (née Hill) and actor Jerry Hardin. She is the sister of Flock CEO Shawn Hardin.

Hardin started her acting career at age 10 in the television series Thunder, and has appeared in over sixty movies and television programs since, including the film Thank You for Smoking, two episodes of Quantum Leap (she played Abigail in parts II and III of "Trilogy"), and one of the Little House on the Prairie movies (in which she played the love interest of Albert Ingalls). In 1988, she starred in the short-lived television adaption of the film Dirty Dancing in Jennifer Grey's role of "Baby". She also has a recurring role as Trudy Monk on the USA Network series Monk. She currently appears on The Office as former corporate manager Jan Levinson.

According to producer Bob Gale's statements on the DVD commentary for Back to the Future, Hardin was originally cast to play the character of Jennifer Parker. When Michael J. Fox was cast as Eric Stoltz's replacement, Hardin was replaced because producers felt that she was too tall to appear onscreen with Fox.

Hardin is also a singer, and has released two albums: "Purr" and "The Meloradrama". In 1991, she sang Begin the Beguine in the film The Rocketeer.

She will play the role of Fantine in the Hollywood Bowl's concert of Les Miserables.

She is married to actor Gildart Jackson and has two daughters, Rory Melora (born 2001) and Piper Hardin Jackson (born 2005).