Her mother spent a lot of timing moving to different communities with her daughter before settling in Albuquerque, New Mexico by the time Kelly was in junior high.
After graduating high school, Kelly returned to Los Angeles and began to work on her modeling career. While on a test shoot with a modeling agency, a former Playboy Playmate expressed interest in managing her. She placed Kelly as a receptionist at a surgeon's office. In exchange for the time she put in, he offered to provide her with breast augmentation. She ultimately refused and was fired shortly after.
Her experience in the medical field prompted her to go to school for a year so she could become a surgeon's assistant. Later, she became a scrub tech. During the four years she did that, she kept auditioning for film and TV spots. 2003 saw the start of Kelly's acting career, as she appeared as an extra in the short film Turbo-Charged Prelude, which was followed by a guest appearance on Cracking Up. Since then, she appeared on television shows such as Entourage, Drake & Josh, and American Dreams. In 2005, she got a three-episode recurring role on the sitcom, What I Like About You.
In April 2006 she was cast as a regular in the NBC series Friday Night Lights, playing the role of Lyla Garrity, one of the cheerleaders. She received much praise for her performance, and left at the end of the third season, as her character went off to college. Also in 2006, she starred in the horror film The Pumpkin Karver as well as a bit role in State's Evidence. In 2007 she teamed up once again with Friday Night Lights creator, Peter Berg for a cameo role in The Kingdom. Kelly dated John Mayer for some time during fall 2007. She also made a cameo in the 2009 film, (500) Days of Summer.
In August 2008, Kelly was cast as the lead in The CW pilot for Body Politic, and despite the praise the show received from critics, The CW did not pick up the pilot. She also appeared in the pilot for the CBS comedy Mad Love. But when the show was picked up, she was replaced by Sarah Chalke. In May 2008, she began dating Derek Jeter until they separated in August 2011.
In April 2010 she starred in the NBC drama, Parenthood, in which she played the role of a behavioral aide of Max, a child diagnosed with Asperger syndrome. She appeared in ten episodes before ending her run with the show in 2011. Also in 2011, Kelly was crowned as "Sexiest Woman Alive" in the Esquire magazine feature. She also took on a role in the Off Broadway play, Love, Loss, and What I Wore from April 27 " May 29, 2011.
In February 2011, Kelly starred in The Roommate, a story about a college freshman whose roommate has an obsession with her that becomes quite violent. She also starred in the reboot of Charlie's Angels, which premiered in September 2011. In spite of the ratings for the premiere, the show received poor reviews from critics across the board, and was cancelled by ABC after the fourth episode due to decreased ratings.
In 2011, Kelly took on the lead role for the comedy, Searching for Sonny, which won "Best Narrative Feature" at the Festivus Film Festival. Also in 2011, she starred with Adam Sandler and Jennifer Aniston in Just Go With It. In December of that year, she went on an eight day United Service Organizations tour through four countries, providing entertainment for US troops. In February 2012, she was one of a few celebrities who modeled red dresses for the Heart Truth Red Dress Collection show, for New York Fashion Week. In June of the same year, it was announced that she would be play the lead in Maroon 5's music video for the song "One More Night." She is also set to star in the 2013 film, The Butler.
It has been said that Kelly has rekindled her relationship with Chris Evans after having dated in 2007.