Photo Courtesy of the Home of Heroes

His parents took him to the United States when he was three weeks old and eventually settled in New York. He attended St. John's College, Fordham, where he was graduated in 1855. To study law he went to Buffalo, thence as a special agent on the post-office to the Pacific coast and was admitted to the bar at Sacramento, California, in 1861.

When the Civil War broke out he raised the first company of cavalry of the Pacific coast, but resigned its captaincy when he found it would not go to the front and went east to Washington where he was appointed an aide-de-camp to General McClellan.

He served with the Army of the Potomac all through the war, and at its close had attained the rank of brevet Major-General of Volunteers. For bravery at the battle of White Oak Swamp he received the Medal of Honor from Congress. In 1866 he resigned from the army and was appointed Corporation Counsel for New York City (1866-67) and then was sent as Minister to Paraguay (1868-69). On his return he practised law until 1881, he was made Receiver of Taxes, U.S. Marshal, State Assemblyman and Senator. In 1896 he was elected Judge of the Court of General Session which office he held at his death.

His brothers, John Eugene, and James Power, were also lawyers and soldiers and both held the command as colonels of the 164th New York Volunteers during the Civil War. John was born in Waterford, Ireland, in 1834, was educated at St. John's College, Fordham, and died at Buffalo, New York, in 1863, from injuries received in the army; James was born in Waterford, 1836, and was killed while leading his regiment at the battle of Cold Harbor, Virginia.

He was serving in the practice of law when he entered the Army at the onset of the Civil War. He served throughout the war, was Chief of Staff of the VI Army Corps and participated in all of the great battles of the Army of the Potomac. He was promoted through the ranks from Private to Major General after being Captain, additional aide-de-camp to General McClellan, October 29, 1862 to February 13, 1863, and Lietuenant Colonel, Assistant Adjutant General, January 1, 1863 to August 15, 1865. In the latter rank, he served as Adjutant General and Chief of Staff in VI Corps under Generals Sedgwick, Franklin and Wright.

He was breveted Brigadier General for Richmond and general war service and Major General, United States Volunteers. He was awarded the Medal of Honor in 1891 for his services at the Battle of White Swamp.

He resigned from the Army in 1866 and became a corporate attorney in New York. He served as United States Minister to Paraguay in 1869; Receiver of Taxes in New York City, 1873-85; United States Marshal, 1885-89; He was a Democrat and was for a time the Manager, National Home for Disabled Volunteer Soldiers, elected by Congress in 1880, 1886, 1892 and 1898, Secretary of the Board of Managers, and then as President of same.

He died at his home in New York in 1906 and was buried in Section 2 of Arlington National Cemetery.

Rank and organization: Captain, and aide_de_camp U.S. Volunteers Place and date: At White Oak Swamp, Virginia, 30 June i862. Entered service at: California. Born: 21 March 1838, Canada. Date of issue: 10 March 1891. 

Citation:

Under fire of the enemy, successfully destroyed a valuable train that had been abandoned and prevented it from falling into the hands of the enemy.

On Wednesday, October 3, at 11:00 am, the Ambassador of the United States of America, James C. Cason, participated in the launching of two stamps to celebrate Peace Corps’ 40th Anniversary and former-U.S. Minister to Paraguay General Martin T. McMahon. The event took place at the Ministry of Foreign Affairs with Ambassador Cason, the Director of the Paraguayan Postal Service, Mr. Atilio Viera, the Vice minister of Foreign Affairs, Antonio Rivas and Vice Director of the Peace Corps in Paraguay, Mr. Jason Cochran.
 
This philatelic initiative allows the dissemination of social and cultural aspects of the bilateral relations between the two countries and the harmonic development of Paraguay.

 General Martin T. McMahon served as United States Minister to Paraguay in 1869, where he showed great interest towards Paraguay’s needs during the Triple Alliance War.

Photos courtesy of Raymond L. Collins