After having appeared fairly regularly in the Worcestershire second team since 2002, Mitchell made his first-class debut for the county in May 2005 against Loughborough UCCE at Kidderminster.
He made only five and 12 as Worcestershire slumped to an eight-wicket defeat, but after a few weeks out of the side Mitchell returned to the first team, and in mid-June he scored 63 against Leicestershire.
Mitchell's first full season was in the Twenty20 Cup saw a failure with the bat – an aggregate of just nine runs in four innings – balanced by five wickets in 13 overs with the ball, including 2–26 against Somerset.
However, Mitchell produced a strong batting performance in the penultimate Championship game of the season against Somerset at Taunton.
He struck 298, his maiden first-class double century and the fifth highest individual score by a Worcestershire batsman, which included 54 boundaries and one six.