Mitchell began playing in the youth ranks of local club Manchester City before joining Football League Second Division side Wigan Athletic in 1999. On 7 December, Mitchell made his senior debut in a 2–1 victory against Burnley in the Football League Trophy First Round. After one season in the youth ranks, he was promoted to the senior squad and marked his 19th birthday with his league debut – a 3–1 win away to Wrexham. Introduced as a 64th minute substitute, Mitchell was dismissed with a straight red card after just eight minutes for a professional foul on Carlos Edwards.