McMAHON, Martin Thomas, soldier, born in Laprairie, Canada, 21 March, 1838. He was graduated at St. John's college, Fordham, New York, in 1855, and subsequently studied law. For a time he was special post-office agent for the Pacific coast, and also served as Indian agent, but at the beginning of the civil war he volunteered and was made captain, becoming aide-de-camp to General George B. McClellan. In 1862 he was appointed adjutant-general and chief of staff of the 6th corps of the Army of the Potomac, under General William B. Franklin, serving also under Generals John Sedgwick and Horatio G. Wright until after the final operations before Petersburg. He resigned in 1866, after receiving the brevets of brigadier and major-general of volunteers on 13 March, 1865. In 1866-'7 he was corporation attorney of the city of New York, and in 1868-'9 he was United States minister to Paraguay. In 1872 he was appointed receiver of taxes in New York city, which office he held until 1885, when he became United States marshal of the southern district of New York. General McMahon received the degree of LL.D. from St. John's college in 1866. During 1886-'7 he was president of the Society of the Army of the Potomac. His brother, JOHN EUGENE, born in Waterford, Ireland, in 1834, died in Buffalo. New York, in May, 1863, and another brother, JAMES POWER, born in Waterford, Ireland, in 1836, killed at, the battle of Cold Harbor, in June, 1864, each had command of the 164th New York volunteers. They had previously graduated from St. John's college, and were practising lawyers when the civil war began.