[1] As a lawyer, McMahon was well known for having defended bank robbers Edwin Alonzo Boyd and his brother Norman in their 1952 trials.
After graduating from Osgoode Hall Law School in 1943, he joined the Royal Canadian Navy for the duration of World War II.
[3] He challenged incumbent reeve Nelson A. Boylen in the 1952 election and defeated him by more than 1,500 votes,[5] running on a promise to attract industry to the municipality, and expand North York's five-member town council which had been the same size since 1923 despite the township's population having increased five-fold, in that period.
[9] He was a member of the Metropolitan Toronto Council's executive committee and supported Metro's by-law fluoridizing the water supply.
[11] In September 1956, McMahon announced that he would not be running again for re-election in order to devote more time to his law practice.