	Mahon played in the club's opening game of the 2003–04 season, a 1–0 home win over Bournemouth in the League Cup, and remained ever-present during the first two months of the campaign.<ref name="Games played by Gavin Mahon in 2003/2004"></ref> An injury in the club's 1–0 away victory against Crewe Alexandra on 4 October 2003 ultimately ruled Mahon out for two months. In January 2004, Mahon scored his first goal for Watford, scoring with a header to give the club a brief lead over Premier League side Chelsea in the Third Round of the FA Cup, in an eventual 2–2 draw. He added to his goal tally twice during the second half of the season, in home matches against Sunderland and Derby County respectively. The goals meant Mahon had scored three times during a season that produced another mid-table finish, making 36 appearances in all competitions. Shortly after the season had finished, he was voted as the club's Player of the Season. Two months into the 2004–05 season, in October 2004, it was announced that Mahon had signed a new three-year contract extension with the club, keeping him at Watford until 2007. On Mahon signing the new deal, Lewington stated — "I've known Gavin a long time and it doesn't surprise me that he's become a very important part of our squad". He made 51 appearances during the season,<ref name="Games played by Gavin Mahon in 2004/2005"></ref> although did not get on the scoresheet, in what was a difficult year for the club. Watford narrowly staved off relegation by two points under the new management of Aidy Boothroyd, after picking up seven points from their final three matches.