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.

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.