Major McMahon served in the 4th & 3rd Va. Regiments, and later, at Fort Recovery in Western Ohio. On June 30, 1794, combined Native American forces from the Shawnee and Miami nations attacked the fort. McMahon was leading a supply convoy back to the fort when he was killed in the attack. A museum now stands at Fort Recovery, Ohio. McMahon is named on a bronze plaque and historical marker at Fort Recovery, where he is buried.