	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.