Kelley was born and raised in a western suburb of Chicago, Glen Ellyn, Illinois. He is the fifth eldest of 15 siblings.
When he was two, his mother took him and five of his siblings to Chicago to meet an agent. Kelley started appearing in commercials and gained his first role in a movie when he was in first grade. Due to his job, he was home schooled for his last two years of high school. When he turned 18, he moved to Los Angeles.
He played guest characters in TV shows like Terminator: The Sarah Connor Chronicles, Law and Order: SVU, Cold Case, Ghost Whisper, and the role of Ryan James in two episodes of Smallville.
Kelley won an Independent Spirit Award for his performance in Mean Creek in 2004.
Also in 2004 he played a lead role in the movie The Dust Factory. In 2006, Kelley appeared in the film Letters from Iwo Jima as a marine. He also played Roy, a great-nephew of Butch Cassidy, in Outlaw Trail: The Treasure of Butch Cassidy.
In 2009, he played the role of Ben Tennyson in Ben 10: Alien Swarm.
That same year, he played a leading role in the movie Prayers for Bobby alongside Sigourney Weaver, which premiered in January 2009 on Lifetime.
In January 2012, Kelley was in another Lifetime TV movie, Sexting in Suburbia, where he played the boyfriend of a suicide victim.
Before being added to the show in Season 3, Kelley auditioned for the role of Scott McCall but lost it to Tyler Posey. He tried once more in Season 2 for the role of Isaac Lahey along side Charlie Carver and Daniel Sharman.