He made his debut in a 4–1 win over Woking at the Abbey Stadium, but struggled to hold down a place in the team. He was an unused substitute in the 2009 Conference Premier play-off Final, as he watched his United teammates lose their second play-off final in as many years. On 16 July 2009, caretaker Cambridge United manager, Paul Carden, announced that McMahon has left the club. He then joined Farnborough for a two-year stay. He enjoyed promotion to the Conference South with the club, as they were crowned champions of the Southern Football League Premier Division in 2009–10. The following year he helped the club to the Conference South play-off final, where they were defeated 4–2 by Ebbsfleet United. In June 2011, he signed for Boreham Wood as a player-coach on a two-year contract. McMahon's stay at Boreham Wood lasted seven months, as in January 2012 he signed for fellow Conference South club Eastleigh for an undisclosed fee. The "Spitfires" went on to post a mid-table finish in 2011–12.