Mitchell spent his first few years in Massachusetts, but when he was 3½ years his family relocated to his father's birthplace of Galway, Ireland where he spent the next 7 years. It was during this time that he made his acting debut at the Town Hall Theatre in a stage production of Oliver!.
In 2004, when Mitchell was 11 years old, his family moved back to the USA and settled in San Diego, California. Less than a year later Mitchell and his mother had moved to Hollywood where he was cast in his first movie role playing a young British boy in the film, The TV Set. Soon after he secured the lead role of Nate Gibbs in the award-winning feature film Country Remedy. He was awarded with the "Best Child Actor in a Supporting Role" at the 2007 International Family Film Festival in Los Angeles, California.