[10] As council leader McMahon sat as one of the 11 members of the Greater Manchester Combined Authority with responsibility for transport.

[11] McMahon was the inaugural chairman of the Co-operative Council Innovation Network and served as the Labour leader of the Local Government Association.

[17][18] McMahon won the selection to be Labour Party candidate at the 2015 Oldham West and Royton by-election following the death of incumbent Michael Meacher.

[23] He supported Owen Smith in the failed attempt to replace Jeremy Corbyn in the 2016 Labour leadership election.

[24] Following Keir Starmer's election as Leader of the Labour Party, McMahon was appointed Shadow Secretary of State for Transport.

[34] In February 2014, McMahon was named "Council Leader of the Year" during the Councillor Achievement Awards hosted by the Local Government Information Unit.

[35] Also in 2014, University Centre Oldham conferred upon him Honorary Fellowship,[36] as well as being elected a Fellow of the Royal Society of Arts (FRSA).

[37] McMahon was appointed Officer of the Order of the British Empire (OBE) in the 2015 Birthday Honours for "services to the community in Oldham",[38][39] and was invested by Prince Charles, on 18 December 2015.