following the departure of previous captain sean dyche, mahon was appointed club captain ahead of the 2005–06 season. watford began the season in positive fashion, with mahon scoring his first goal of the new campaign in a 3–1 home win over burnley on 20 august 2005. he added to his tally in november that year when he scored watford's first goal in a 2–1 away victory against hull city, before also giving watford an early lead in a comfortable 3–1 win against southampton at st mary's in march 2006. watford exceeded all expectations during the season, with mahon's leadership and combative nature playing a key role in guiding watford's young squad to a third-place finish. mahon played in all three play-off matches as watford firstly disposed of crystal palace 3–0 on aggregate, before defeating leeds united by a 3–0 scoreline in the final at the millennium stadium in may 2006. the victory meant watford had earned promotion to the premier league, in what was a memorable first season as captain for mahon, making 43 appearances.<ref name="games played by gavin mahon in 2005/2006"></ref> shortly after gaining promotion, in july 2006, mahon signed a two-year contract extension with the club. he subsequently started in the club's first game since their return to the top flight, playing the whole match as watford lost 2–1 to everton at goodison park. mahon made 38 appearances during the season,<ref name="games played by gavin mahon in 2006/2007"></ref> scoring once with a "fierce volley" in a 4–2 win against portsmouth, a goal that was nominated for april's goal of the month award. watford were relegated back to the championship after finishing in 20th position. the club made the fa cup semi-final that campaign, losing 4–1 to manchester united at villa park, with mahon playing in three of watford's five fa cup ties that campaign.