From St Kevin's via Western U18, the gutsy Mahoney tapered off dramatically for the Magpies late in 1998 after having looked like he was capable of holding down a regular spot. His practice of wearing a helmet was brought about by his fearless approach. Mahoney made an impression as a dogged and courageous defender in his early games for the Bulldogs, but did not command a regular place after that. He played for Williamstown in the VFL and was a surprise selection by Port Adelaide with the final selection in the pre-season draft. He had only a couple of senior chances in the first two months of 2004, and did nothing spectacular, but then grabbed a position and his premiership medal for 2004 was one of the year's great stories. Injuries hit him early in 2005 but he finished strongly and in 2006 lifted a cog to produce his best year.