John Sedgwick
At the outbreak of the Civil War, 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 lieutenant colonel. He served as aide de camp to William B. Franklin (May 1862- January 1863) and Chief of Staff and Assistant Adjutant General of the 6th Corps (1 January 1863 to August 1865), serving under John Sedgwick and Horatio G. Wright. McMahon was with 6th Corps commander John Sedgwick at Spotsylvania when Sedgwick was killed. McMahon was the author of "Maj. Gen. John Sedgwick - In Memoriam" (1885).