John Joseph McMahon (April 22, 1875 – September 8, 1958)^[1] was an American architect who even today remains highly regarded for his churches, schools and other buildings for Catholic clients in Connecticut, especially Hartford and New Haven.

McMahon was born April 22, 1875, in Hartford, Connecticut, where he would live for the rest of his life. He studied as a boy at St. Patrick School where one of his classmates, John F. Callahan, would later become a Catholic priest and one of McMahon's clients. He left school a few months before graduation in 1890 and took a job as an errand boy.

Three years later he was hired by the architectural firm of Frederick R. Comstock as an apprentice. He worked both in Hartford and New York City. His most notable project was the 1899 Second Church of Christ Scientist, located at 68th Street and Central Park West, New York City.^[2] His architectural training was cut short when he enlisted in the Connecticut National Guard to take part in the Spanish–American War. Eventually McMahon attained the rank of colonel and was henceforth known, professionally and otherwise, as "the Colonel".

In 1900 McMahon became associate architect of the Hartford, CT firm J. J. Dwyer & J. J. McMahon, with John J. Dwyer. He remained in this position until 1911 when he entered into a long partnership with Frank Warren Whiton, forming the firm of Whiton and McMahon. They continued together until 1932 when the practice was dissolved due to the economic depression. After that McMahon practiced under his own name and briefly with architect Russell Hills.