John McMahon was born on July 15, 1912, in Rochester, New York. He entered the U.S. Naval Academy on June 30, 1930, and was commissioned an Ensign in the U.S. Navy on May 31, 1934. His first assignment was as officer-of-the-deck at sea aboard the battleship USS California (BB-44) from July 1934 to May 1936, followed by service aboard the heavy cruiser USS Astoria (CA-34) from May to June 1936. He next attended Submarine School at Submarine Base New London, Connecticut, from June to December 1936, and then served aboard the submarine USS R-13 (SS-90) from December 1936 to June 1937. LtJg McMahon served aboard the submarine USS R-2 (SS-79) briefly from June to July 1937, followed by service as Assistant Engineering Officer and Gunnery and Torpedo Officer aboard the submarine USS Permit (SS-178) from July 1937 to November 1941. LT McMahon joined the crew of the submarine USS Grunion (SS-216) during her fitting out at the Electric Boat Company in Groton, Connecticut, in January 1942, and remained aboard her after her as Engineering and Dive Officer after her commissioning on April 11, 1942. He was killed in action during a confrontation with the armed Japanese freighter Kano Maru on July 30, 1942. On August 22, 2007, a search team organized by the three sons of CDR Mannert Abele (the Captain of the Grunion when she was sunk) used a remotely operated vehicle to find a sunken vessel 3,000 feet down in the Bering Sea north of Kiska Island at the tip of the Aleutian Islands. On October 1, 2008, the U.S. Navy announced that the sunken vessel is the World War II submarine USS Grunion (SS-216).

His Navy Commendation Medal Citation reads:

For meritorious conduct as Engineer Officer of the U.S.S. GRUNION which destroyed three enemy destroyers while engaged in a war patrol in enemy controlled waters. Despite severe and persistent anti-submarine measures resulting from these three successful attacks, the GRUNION was brought safely through the counter attacks and continued an aggressive war patrol. As Engineer Officer of the GRUNION, your performance of duty was an important and material contribution to the prosecution of this war.