McMahon was appointed by the House of Representatives as a manager to conduct impeachment proceedings against Secretary of WarWilliam W. Belknap.
He was unsuccessful for re-election in 1880.
Later career
He returned to private law practice in Dayton. He served as the president of the Ohio State Bar Association in 1886,[3] and was a losing candidate for the Senate in 1889.
McMahon was married January 23, 1861, to Mollie R. Sprigg, of Cumberland, Maryland.[1] They had a son, J. Sprigg McMahon, and daughter, Louise McMahon.[4]