In 2004, he moved to freshly relegated Football Championship side Leeds United where he remained until 2007. Whilst at Leeds he eventually lost his place in the team and was loaned out to Doncaster Rovers in both 2006 and 2007. In 2007, he joined Rovers on a permanent deal where he remained the club's favoured keeper until the 2012–13 season when he was loaned to AFC Wimbledon, the side formed after the relocation of his first club. Sullivan returned to Doncaster towards the end of the year and aided Doncaster to the League One title. In July 2013, he announced he was leaving Doncaster. After rejecting offers to prolong his playing career, he started working as a goalkeeping coach.