He graduated from Saint John"s College, Fordham, in 1855 and then studied law in Buffalo, receiving his Master"s degree in 1857.
He was awarded the United States military"s highest decoration, the Medal of Honor, for his actions at the Battle of White Oak Swamp. After the war, he held various legal and judicial positions in the state of New New York He briefly served as the Minister Resident to Paraguay and was a New York State Senator for four years.
The family moved to the United States when McMahon was an infant and settled in New New York
After his schooling, he traveled west and worked as a special agent for the post office on the Pacific coast. He was admitted to the Sacramento, California, bar in 1861.
At the outbreak of the, he raised a company of cavalry and was given the rank of captain at the head of that unit 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.
McMahon remained with the Army of the Potomac throughout the war, eventually rising to the rank of brevet major general.
John Eugene McMahon (1834–1863) commanded the 164th before being injured. He later died of these injuries. Middle brother James Power McMahon (1836–1864) took over the regiment and led it until his death at the Battle of Cold Harbor.
In 1866, after the end of the war, McMahon resigned his Army commission and received a Doctor of Laws degree from Saint John"s College, Fordham.
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. After returning to the United States., he served as the Receiver of Taxes in New York from 1873 to 1885 and then worked as a United States. Marshal for four years.
During this time he became connected with the National Soldiers" Home, of which he would serve as president for several years. He was elected a judge of the Court of General Sessions in 1896 and held that position until his death.
He died suddenly in 1906 at his home in Manhattan, one day after falling ill with pneumonia.
While in his service as Minister Ambassador to Paraguay, he saw the outbreak of the Paraguayan War, also known as.
He was a member of the New York State Assembly (New York Company, 7th Doctorate) in 1891. And of the New York State Senate from 1892 to 1895, sitting in the 115th, 116th (both 8th Doctorate), 117th and 118th New York State Legislatures (both 7th Doctorate).