In the summer of 2008, Mangan signed for Forest Green Rovers in the Conference Premier. It was at Forest Green that Mangan's goalscoring reputation began to form; his first season saw him finish as the league's top scorer with 26 goals in 41 games.
During the summer of 2008, Mangan joined Forest Green Rovers in the Conference Premier. It was during his time with Forest Green that he established himself as a prolific goal scorer; his debut season saw him emerge as the league's top scorer with 26 goals in 41 matches.