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.

Injuries hampered the latter stages of his QPR career, and he was briefly loaned out to Crystal Palace in March 2011, although he did not make any appearances for the club.

Mahon left Notts County at the end of the season, and briefly joined Portsmouth in October 2013 for a one-month spell.

[3] In July 1996, he signed for Hereford United of Division Three on a free transfer,[4] making his first professional appearance in the club's 1–0 victory against Doncaster Rovers on 24 August 1996.

[7][8] In November 1998, Mahon signed for Division Three club Brentford, joining for a nominal fee of £50,000, which eventually increased to £130,000 under the terms of the deal.

[9] Mahon debted for Brentford in the club's 2–1 defeat to Leyton Orient at Brisbane Road on 21 November 1998, and played regularly in central midfield.

[19] 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.

[25][26] Shortly before the start of pre-season, Watford stated that Mahon was going to miss the opening months of the new season after sustaining a knee injury in May 2002.

[36][37] The goals meant Mahon had scored three times during a season that produced another mid-table finish,[38] making 36 appearances in all competitions.

[40] 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".

[49] He subsequently started in the club's first game since their return to the Premier League, playing the whole match as Watford lost 2–1 to Everton at Goodison Park.

[21] Mahon subsequently joined Championship rivals Queens Park Rangers on an 'emergency loan' basis on 31 December 2007, with a view to a permanent transfer at the end of the season.

[25][59] Although disappointed by his exit at Watford, Mahon stated it was "a very exciting time to be joining" QPR, and that "the ambition and drive of the board and the new investors really impressed me and I'm delighted to be here".

Mahon promptly scores a last gasp winner and Briatore, his football genius now proven, erupts in a mixture of self-righteous anger and joy in the directors' box".

[62][66] Mahon started in QPR's opening game of the 2009–10 season on 8 August 2009, playing the whole match as the club drew 1–1 at home to Blackpool.

[70] With Mahon's contract expiring at the end of the season, manager Neil Warnock stated – "He'll probably come back and do a pre-season with us and try and show me what he can do.

[3][81] Two weeks into the new season, on 25 August 2011, Mahon signed for League One club Notts County on an initial pay-as-you-play deal.

[85][86] He was described as a "pivotal" player in Notts County's season,[87] and went on to make 34 appearances as they missed out on a play-off place on goal difference.

[88][89] At the end of the season, Notts County stated they wanted to retain Mahon's services, and the player was offered a new one-year contract,[90][91] which he signed on 28 June 2012.

[94] In November 2012, it was revealed that Mahon was the subject of loan bids from two League Two clubs in the form of Bristol Rovers and Port Vale.

[95] He rejected the opportunity of going out on loan after Notts County manager Keith Curle told him there was still of chance of him regaining his first-team place.

[95] However, two weeks later, Mahon stated he was prepared to leave Meadow Lane if he did not play more games[95] – "You do think like that sometimes, when you travel halfway across the country to get splinters in your backside, but that's football, you have to wait for your chance to come.

[100][101] He made his Portsmouth debut four days after signing, playing the whole match in a 2–1 Football League Trophy away win at Oxford United.

[102] Mahon was sent-off in a 2–1 defeat to former club Stevenage in an FA Cup tie on 9 November 2013, receiving a straight red card for an elbow on Filipe Morais.

[105] Ahead of the 2014–15 season, Mahon was invited to spend time on trial at Barnet,[108] and he played in a pre-season friendly fixture against Peterborough United in July 2014.