Mitchell began his professional hockey career when he was drafted in the first round, nineteenth overall by the Plymouth Whalers in the 2001 Ontario Hockey League Draft. Mitchell had a distinguished career with the Whalers, as he was named captain of the team in his final year of junior hockey.
Mitchell was then drafted in the fifth round, 158th overall by the Maple Leafs in the 2003 NHL Entry Draft. Mitchell was then signed to a minor-league contract, where he played his first three professional seasons with the Toronto Marlies, the Maple Leafs' minor leage affiliate in the American Hockey League. Mitchell was then brought up to the Maple Leafs' professional roster before the 2008-09 season. On November 1, 2008, he scored his first two career goals against Stephen Valiquette of the New York Rangers in a 5–2 Maple Leafs victory. He was also selected as the game's first star. However, in a November 6, 2008 game against the Boston Bruins, Mitchell suffered his first injury of his professional career, as he suffered a shoulder separation that removed him from the Maple Leafs' active roster for a week.