After his father died when Monaghan was four years old,[6] Monaghan's mother had difficulties raising him alone, and at age six, in 1943, Monaghan and his younger brother ended up in an orphanage until their mother collected them again in 1949.[7] The orphanage, St. Joseph Home for Children in Jackson, Michigan, was run by the Felician Sisters of Livonia; one of the nuns there inspired his devotion to the Catholic faith and he later entered St. Joseph's Seminary, in Grand Rapids, with the desire to eventually become a priest. Subsequently, he was expelled from the seminary for a series of disciplinary infractions.[5]
In 1956, Monaghan enlisted in the United States Marine Corps by mistake; he had meant to join the Army.[7] He received an honorable discharge in 1959.