McMahon was an amateur at Kooyonga Golf Club in Adelaide until early 1926, giving up a position as a government surveyor to become a professional golfer.

[3] A professional event was organised immediately after the Open but McMahon failed to finish in the top 4 in the qualifying stage.

[5] The South Australian Professional Championship was first played in 1927 and McMahon finished second behind Rufus Stewart.

[9] The Australian Open was played at Royal Adelaide again in 1929 and McMahon improved on his previous best, finishing fourth.

[10] When it returned to Royal Adelaide in 1932 he improved again, finishing runner-up and leading professional, a stroke behind Mick Ryan.