| Honorific-Prefix: | His Excellency, The Most Reverend |
| John Joseph McMahon | |
| Bishop of Trenton | |
| Term: | 1928-1932 |
| Ordination: | May 20, 1900 |
| Consecration: | April 26, 1929 |
| Consecrated By: | William Turner |
| Birth Date: | 27 September 1875 |
| Birth Place: | Hinsdale, New York, US |
| Death Place: | Buffalo, New York, US |
| Religion: | Roman Catholic |
| Education: | St. Bonaventure College Pontifical Urbaniana University |
| See: | Diocese of Trenton |
| Predecessor: | Thomas J. Walsh |
| Successor: | Moses E. Kiley |
John Joseph McMahon (September 27, 1875 - December 31, 1932) was an American prelate of the Roman Catholic Church. He served as bishop of the Diocese of Trenton in New Jersey from 1928 until his death in 1932.
John McMahon was born on September 27, 1875, in Hinsdale, New York. He graduated from Union High School in 1893.[1] McMahon then attended St. Bonaventure University in Allegany, New York, earning a Bachelor of Arts degree. He completed his theological studies at the Pontifical Urbaniana University in Rome.
McMahon was ordained to the priesthood for the Diocese of Buffalo on May 20, 1900.[2] On returning to New York, he served as a curate in parishes in Jamestown and Buffalo before becoming pastor of a parish in Newfane. He also served as assistant superintendent of diocesan schools and director of the Holy Name Society.
On March 2, 1928, McMahon was appointed the fourth bishop of the Diocese of Trenton by Pope Pius XI. He received his episcopal consecration on April 26, 1928, from Bishop William Turner, with Bishops Thomas Walsh and Edmund Gibbons serving as co-consecrators, at St. Joseph's Cathedral. He was installed at St. Mary's Cathedral, Trenton, New Jersey on May 10, 1928.
After four years as bishop, John McMahon died in Buffalo at age 57 on December 31, 1932.