mahon started his career as a trainee at wolverhampton wanderers, although he did not make any first-team appearances for the club. in july 1996, he joined hereford united on a free transfer, and went on to play two-and-a-half years of regular first-team football. mahon signed for brentford in november 1998, for a fee that rose to £130,000. he helped the club earn promotion from division three during the 1998–99 campaign, and went on to make over 150 appearances for the west london side. he joined watford for £150,000 in march 2002, and would later captain the club to promotion into the premier league.