Garscadden, Glasgow born full back Johnny McMahon began his senior football career with Scottish League Clyde in 1899 before joining First Division Preston North End, for whom he made his Football League debut against Manchester City in November 1900. He established himself as a first team regular, however Preston were relegated in his debut season. Having scored once in 71 appearances for the Deepdale club, he transferred to Manchester City in December 1902, playing 17 games as the won the Second Division Championship in 1903.
He was then part of the City team which defeated Bolton Wanderers 1-0 to win the 1904 FA Cup Final at The Crystal Palace, undoubtedly the highlight of his career, being a key part of the team that also finished as League runners up in 1904 and in third place in 1905. In the close season of 1906, after a single goal in 109 appearances for The Citizens, he moved to Bury, where he played 4 seasons, making 63 appearances, until his retirement from the game in 1910.