He was appointed as manager of English Football League side Macclesfield Town in August 2019 but tendered his resignation in January 2020 amidst a financial crisis at the club.
He took charge at Hornchurch in January 2024 and led the club to the Isthmian League Premier Division title at the end of the 2023–24 season.
In need of first-team experience, he joined League Two Torquay United on loan in March 2004,[5][6] making his debut as a late substitute for Martin Gritton in the 2–2 home draw with Yeovil Town on 3 April 2004.
[7] He was released by West Ham at the end of the 2003–04 season and joined Martin Foyle's League One Port Vale on non-contract terms in September 2004.
He dropped down a division to join Leyton Orient on a free transfer two months later, after rejecting a further short-term deal at Port Vale.
[10] 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.
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.
[20] 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.
He was named as Manager of the Month for January 2017 after Ebbslfeet won all four of their league games against Dartford, Chelmsford City, Poole Town and Wealdstone.
[34] However, they made up for their previous season's defeat and justified their 96-point finish after coming from 1–0 down with ten men to record a 2–1 victory over Chelmsford City in the play-off final at Stonebridge Road.
[40] On 19 August 2019, McMahon was appointed as manager of EFL League Two club Macclesfield Town, succeeding Sol Campbell.
[41] He got off to a good start on the pitch, however, the "Silkmen" squad released a statement to say they had gone unpaid for the month of September as HM Revenue and Customs delivered the club a winding up petition.
[42] He resigned on 2 January 2020 after financial problems resulted in players striking, which left the club unable to fulfill fixtures and punished with a six-point deduction.
[48] Dagenham ended the 2021–22 season in eighth-place, missing out on the play-offs by a single point despite beating runners-up Wrexham on the final day.