Thornton was born in College Station, Texas. She made her television acting debut in the pilot episode of the Fox sitcom Quintuplets. She then went on to appear in 8 Simple Rules, American Dreams, The O.C., Desperate Housewives, That's So Raven, Jericho, Wizards of Waverly Place and Hannah Montana.
In 2009, she began her role as Tawni Hart on the Disney Channel Original Series, Sonny with a Chance and then went on to play a part in the Disney Channel Original Movie Hatching Pete. Sonny With a Chance ended on January 2, 2011 and was replaced by So Random!, which premiered on June 5, 2011, with Thornton in a starring role.
Thornton is a Christian. In December 2009, Thornton became engaged to her court officer boyfriend, Christopher Carney. The couple was married on November 12, 2011 at Garvan Woodland Gardens, in Hot Springs National Park, AR. Demi Lovato served as one of the bridesmaids in Thornton's wedding, and caught the bouquet. On February 13, 2012, Thornton announced that she was pregnant with their first child. The couple welcomed their son, Kenneth James Carney, on August 14, 2012.