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. QPR beat "four or five other Championship sides" to his signature. Watford received an initial payment of £200,000 for Mahon, with an additional £50,000 due on further clauses. Although disappointed by his sudden 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". He made his debut a day after signing, coming on as a 67th-minute substitute in the club's 3–1 win over Leicester City at Loftus Road. Mahon went on to score his first goal for the club on 12 February 2008, netting the opening goal of the match as QPR squandered a two-goal lead to lose 4–2 at home to Burnley. He made 17 appearances during the second half of the season, Mahon scored his first goal of the season in a 1–0 home victory over Cardiff City in November 2008, coming on as a 67th-minute substitute, before scoring a headed goal against a nine-man Cardiff side with just 10 minutes remaining. The goal was highlighted in the BBC documentary "The Four Year Plan", as joint-owner Flavio Briatore ordered sporting director Gianni Paladini into "going down to the touchline to order caretaker manager Gareth Ainsworth to bring Gavin Mahon off the bench. 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". His second goal of the season came in April 2009, scoring with a diving header to equalise proceedings after QPR had trailed by two goals to Sheffield Wednesday — with Mahon atoning for his earlier own goal. QPR went on to win the match 3–2, all of their goals coming in the last half-hour of the match. He played under three different managers during the season, making 40 appearances as the club finished in mid-table.