John A. MacMahon appears in the Philadelphia city directories sporadically from 1889, when he lists himself as an architect in the Drexel Building, to 1912, when he appears without an office address. In the city directories of 1910 and 1911 he adds that he is associated with the firm of Cope & Stewardson, and this is confirmed by a photograph which appears in the 1923 T-Square Club exhibition catalog, in which the office of Cope & Stewardson for the year 1899 is published.

MacMahon joined the T-Square Club in 1885.

Written by Sandra L. Tatman.