After serving as a
school trustee, he was elected to North York Township Council as deputy reeve for Ward 3 in the January 1950 municipal election, and then re-elected for a second term at the end of the year. 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. As reeve, McMahon supported the
Ontario Municipal Board's recommendation that North York and the 11 other suburbs of the
Old City of Toronto be federated with Toronto into a new municipality of
Metropolitan Toronto rather than amalgamated into a unitary city. He later said the township had been close to
bankruptcy and that joining Metropolitan Toronto had saved it from that fate. In 1953, McMahon was
acclaimed to a second term as reeve, and was subsequently re-elected in
1954 and
1955. He was a member of the Metropolitan Toronto Council's executive committee and supported Metro's by-law
fluoridizing the water supply. He was an active member of the
Liberal Party but was defeated by 1,100 votes. 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. In 1960, a group of ratepayers accused McMahon of having illicitly purchased public land while serving as deputy reeve nine years earlier, using a secretary in his law office as an intermediary and then purchasing it from her several weeks later for a nominal amount. Township officials stated that the township sold the land for a "fair price" and no laws were broken. ==Legal career==