in november 1998, mahon signed for division three side brentford, joining for a nominal fee of £50,000, which eventually increased to £130,000 under the terms of the deal. mahon made his brentford debut in the club's 2–1 defeat to leyton orient at brisbane road on 21 november 1998, and instantly became a permanent fixture in the side's midfield.<ref name="games played by gavin mahon in 1997/1998"></ref> he opened his goalscoring account for brentford in the club's first match of 1999, scoring the third goal in a 3–1 win over barnet at griffin park. his first season with the club proved to be a successful one, with brentford earning promotion to division two as champions, winning the league by four points. mahon made 32 appearances during the campaign, contributing with four goals. the 1999–00 season was mahon's first full season with the west london club, making 44 appearances in all competitions and scoring three times,<ref name="games played by gavin mahon in 1999/2000"></ref> as brentford consolidated their place in division two with a mid-table finish.