He was awarded the United States military's highest decoration, the Medal of Honor, for his actions at the Battle of White Oak Swamp.

McMahon was born in La Prairie, Lower Canada,[1] to a family of recent migrants from Waterford, Ireland.

After learning that his company would not be sent to the front lines, he resigned his command and returned east, where he was appointed aide-de-camp to General George B. McClellan.

Decades after the end of the conflict, on March 10, 1891, he was awarded the Medal of Honor for his actions at the Battle of White Oak Swamp on June 30, 1862.

Middle brother James Power McMahon (1836–1864) took over the regiment and led it until his death at the Battle of Cold Harbor.

[6] After the war, he was elected as a companion of the New York Commandery of the Military Order of the Loyal Legion of the United States.

[1] He was New York City's corporation counsel for two years before becoming the United States minister to Paraguay, a position he held from 1868 to 1869.