Kelly is the only child of former Aerosmith guitarist Rick Dufay and the late Maureen Kelly, a former Vegas showgirl. According to Kelly, her childhood was "pretty rough". She resented Dufay, as he had abandoned her and her mother. However, at seventeen, she decided to try to reconcile with him by inviting him out to New Mexico. She is of Irish and French descent.
After graduating from high school, Kelly returned to Los Angeles with her father to pursue acting as a hobby while she went to school to become a surgical technician. However, after her first acting class she got "hooked" and began pursuing acting as a career. Success came slowly, and she began her career with small roles in various movies and shows, including the film State's Evidence, and a recurring-guest role on the TV program What I Like About You.
Kelly dated Alex Galvin in 2003 and Dylan Brazil in 2005, and had a relationship with actor Chris Evans. She dated musician John Mayer from September 2007 to January 2008.
It has been reported that Kelly is dating Yankee short stop, Derek Jeter. It is rumored that the couple is engaged. Neither of the two have confirmed either rumor.