 He turned professional in July 2004, when Stanley went full-time after their first season in the Conference, during which Mullin scored 24 goals in all competitions and won the club's player of the year award.On 14 April 2007, Mullin broke Chris Grimshaw's record for club appearances for Accrington Stanley with 362 games, in a 4–1 victory against Grimsby Town.He scored the first of Stanley's four goals during the match.