John A. McMahon (February 19, 1833-March 8, 1923) was a United States Representative from Ohio. He was the nephew of Clement Vallandigham, another Representative from Ohio.
McMahon was born in Frederick County, Maryland, and graduated from St. Xavier College in 1849. He studied law and was admitted to the bar in 1845; he would begin practice in 1854. He was elected to the Forty-fourth, Forty-fifth, and Forty-sixth United States Congresses, from 1875 until 1881.
He was appointed by the House of Representatives as a manager to conduct impeachment proceedings against Secretary of War William W. Belknap. He was unsuccessful for re-election in 1880 and returned to private law practice in Dayton. He served as the president of the Ohio State Bar Association in 1886, and was a losing candidate for the Senate in 1889. He died in Dayton and was buried in Woodland Cemetery.
Preceded by: Lewis B. Gunckel U.S. Representative from Ohio's District 4 1875 - 1879 Succeeded by: J. Warren Keifer Preceded by: Mills Gardner U.S. Representative from Ohio's District 3 1879 - 1881 Succeeded by: Henry Lee Morey