During this time, he played Gaelic Football for Garda GAA and reached the Dublin County Championship Final in 1923 where they lost by a point to O'Tooles.
He added another League winners medal in 1929/30 as Bohs great amateur team of that time showed their class.
McMahon would also score in the winner in a 3-2 victory over Standard Liège that would win Bohemians the Aciéries d'Angleur tournament.
The following October, he captained them against the Welsh League XI and scored the first goal (a penalty) in a 4-3 win for his side.
Johnny unfortunately had to withdraw from the following squad due to an injury picked up when playing for Bohs against a Peru/Chile XI and would never receive another cap.