Smart was born in Topanga, California to John Smart (a salesman) and Judy Carrington (who worked at the Getty Center in Los Angeles); she has a brother, Adam. Inspired by her friend (actress Vinessa Shaw), Smart studied ballet for ten years and enrolled in acting classes when she was 16, but began her career as a model, working in Italy, France, Mexico and Tahiti.

Smart had a very minor role in the 1997 film Starship Troopers as the co-pilot for (and friend of) Carmen Ibanez (Denise Richards). In 1999, Smart played the girlfriend of a popular football player (played by James Van Der Beek) in the film Varsity Blues. Also, Smart appeared in the series Felicity, as a girlfriend of Scott Foley's character. She subsequently appeared in a number of films catering to a teenage audience, many of which were successful, including Road Trip (2000) and The Butterfly Effect (2004). In 2002, she was ranked #27 in Stuff magazine's "102 Sexiest Women in the World" and in 2004, she was nominated for "Best Kiss" at the MTV Movie Awards for her role in Starsky & Hutch.

In 2005, Smart co-starred with Ryan Reynolds in the romantic comedy film Just Friends, which did fairly at the box office, grossing $32 million.

She also had a small role in American sitcom Scrubs, playing the wife of a coma patient. She had been nicknamed TCW (Tasty Coma Wife) by JD and his friends. JD has a short relationship with her following the death of her husband.

Smart will be a regular cast member in the new CBS television series Smith.

Smart was the speaker for the "Heal the Bay" organization; she has also worked with the Humane Society and the "Environmental Media Association", and was named one of "Organic Style" magazine's "Women with Organic Style" of 2004.