He signed professional forms with United in March 2005. On 31 December 2005, Goodwillie made his competitive debut for United, appearing as a late substitute in the defeat to Rangers at Ibrox. On 4 March 2006, in his fifth appearance for United, Goodwillie came off the bench to score a late goal for United against Hibernian at Easter Road. During the 2005–06 season, Goodwillie made ten substitute appearances for the first team, with the majority coming in the final few minutes of each match.