Dublin-born Mahon, who was educated at St. James Street Christian Brothers Secondary School in the Irish capital, joined English Football League club Tranmere Rovers in 1995. In five years at the club, Mahon made a total of 144 appearances, scoring fourteen goals. He played in the 2000 Football League Cup Final as Tranmere lost 2–1 to Leicester City at Wembley on 27 February 2000.