Teammate Dean Smith had set up the move to Brisbane Road and would himself join Orient soon after. Leyton Orient After impressing on a non-contract basis, he signed a six-month deal with Leyton Orient in January 2005. He played 33 games of the "O's" promotion winning campaign of 2005–06 but fell out of favour the following season and joined Notts County, back in League Two, on loan in November 2006. After his loan spell finished the following January, McMahon joined Conference National club Stevenage Borough. Non-league A fairly regular member of the first team at Broadhall Way, he was a sub in the FA Trophy final at Wembley. In May 2008 he signed a two-year contract with the club. On 26 November 2008, he was transfer listed by Borough, along with teammate John Martin, and in the January 2009 transfer window McMahon agreed to join fellow Conference Premier club Cambridge United. 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. He was sold to league rivals Dover Athletic for an undisclosed fee in September 2012. Eastleigh manager Richard Hill had rejected two bids from the "Whites", before Dover boss Nicky Forster put in a bid that Eastleigh director Stewart Donald felt "matched our valuation" and represented "good value". He played in the 3–2 play-off final defeat to Salisbury City on 12 May 2013. In June 2013, he signed for Ebbsfleet United for a fee of £13,000. He became