 At the end of the 2007–08 season, Sullivan signed a new one-year contract with the club.It was announced on 12 May 2009 that Sullivan would be leaving Brighton after the expiry of his contract at the end of June 2009.Sullivan agreed to join League One rivals Millwall on 2 June 2009, and made his debut on 1 September 2009 against Barnet in the Football League Trophy.