McMahon is a member of the Ballymun Kickhams Senior Football team. in 2013, Ballymun reached the 2012–13 All-Ireland Senior Club Football Championship final. McMahon scored a goal as Ballymun were defeated by St. Brigid's of County Roscommon by a single point, on a scoreline of 2-11 to 2-10.
Inter-county
McMahon won the 2008 O'Byrne Cup with Dublin, defeating Longford in the final. He made his Championship debut that year as a sub against Louth. He played Interprovincial Championship Football for Leinster and won 2 Dublin Under 21 Football Championships with Ballymun. He was named on the 2010 GPA Gaelic Team of the Year. He replaced James McCarthy as a sub in the second half of the 2011 All-Ireland Senior Football Championship Final, a game which Dublin won by 1-12 to 1-11 against Kerry. On 22 September 2013, McMahon made his first start of the Championship in the All Ireland Final. He was a key performer in Dublin's defeat of Mayo on a scoreline of 2-12 to 1-14. McMahon helped Dublin to win the 2015 All Ireland final. Throughout the season he became a strong attacking threat despite being named at left corner back. He scored 1-02 in the replayed semi final against Mayo, notably holding Aidan O'Shea to a single point in over both games. McMahon was a key performer in the final where Dublin defeated reigning champions Kerry 0-12 to 0-09, with McMahon scoring a point in the 1st half while keeping marker Colm Cooper scoreless. McMahon received his first All Star award while he was also named on the shortlist for GAA Footballer of the Year, ultimately won by teammate Jack McCaffrey. McMahon was again a key performer in 2016 as Dublin again defeated Mayo by a single point after a replay on a scoreline of 1-15 to 1-14 to retain the Sam Maguire Cup. McMahon received his second All Star for his performances. In 2017 Dublin were narrowly defeated in the National League final by Kerry by a single point. They then went on to win a record 7 Leinster titles in a row. On 17 September, McMahon was heavily involved as Dublin claim a historic 3 in a row All Ireland titles with another narrow 1-17 to 1-16 victory against Mayo. Having been outplayed in the first half, the Dubs turned the game around to win a thrilling game courtesy of a 75th minute Dean Rock free.
In November 2012, McMahon was the Strength and Conditioning Coach at Shamrock Rovers. McMahon is a business owner and operates multiple gyms. He also launched a health food company. In 2017, McMahon released an autobiography entitled "The Choice", with a strong emphasis on the story of his older brother John who died in 2012 as a result of drug addiction. It was named Eir Sports' book of the year. He married his long-term girlfriend Sarah Lacey in December 2019.
Charity
He has set up a charity entitled the half time talk in order to educate, train and empower young, unemployed adults in communities nationwide. In 2017 after winning his fifth All-Ireland Senior Championship, McMahon released his autobiography called The Choice, which was co-written by sports journalist Niall Kelly. The book won the 2017 Eir Sports Book of the Year award in December 2017. The book also won the Sports Book of the Year award at the 2017 Irish Book Awards.
