in march 2002, mahon signed for division one side watford for an initial fee of £150,000. the move came about as a result of watford reserve team manager ray lewington, who had previously signed mahon at brentford, recommending the player to new manager gianluca vialli. vialli subsequently went to watch him play, and opted to make mahon his first signing for the club. on signing for watford, mahon stated — "as soon as i knew watford were interested i was ready to jump at the chance". he made his watford debut on 9 march 2002, playing the whole match in a 2–0 away win over crystal palace. mahon went on to make a further five appearances for the club during the remainder of the 2001–02 season. ahead of the 2002–03 season, under the management of lewington once again following vialli's sacking, mahon was one of several players who took a voluntary pay cut as a result watford's financial troubles. shortly before the start of pre-season, it was announced that mahon was going to miss the opening months of the new campaign after sustaining a knee injury in may 2002. he returned to first-team action in the club's 2–1 win over burnley at vicarage road on 30 november 2002, and went on to make a total of 22 appearances during the campaign.<ref name="games played by gavin mahon in 2002/2003"></ref> five of those appearances were in watford's fa cup run, as they narrowly lost 2–1 in the semi-final to southampton at villa park.