Career
McMahon joined the Army from Portland, Maine in 1967, and by March 19, 1969, was a Specialist Four serving as a combat medic in Company A, 2nd Battalion, 1st Infantry Regiment, 196th Infantry Brigade, Americal Division. On that day, in Quang Tin Province, Republic of Vietnam, he attempted to rescue three wounded soldiers despite heavy enemy fire. He was able to carry two of the men to safety but was mortally wounded while trying to rescue the third.
McMahon, aged 20 at his death, was buried in Mount Hope Cemetery, Lewiston, Maine.
A Lewiston elementary school is currently named in his honor.