He studied theology with a view to becoming a priest, but the local bishop did not ordain him because there were too many priests in that region at the time. John came to New York in 1836 and was ordained to the priesthood by the bishop of that diocese. In 1840, he joined the Redemptorists and became its first member to make a religious profession in the United States.

He continued his missionary work in Maryland, Virginia and Ohio. He was ordained bishop of Philadelphia in 1852 and organised the parochial school system into a diocesan one, increasing the number of pupils within a short time. Many migrants from Europe arrived in the United States at that time and he learned several languages, including Irish, to attend to them. He died suddenly on 5 January 1860 and was buried in St. Peter the Apostle Church in Philadelphia. He was canonised by Pope Paul VI in 1977

His Feast day is celebrated on 5 January and he is the Patron Saint of Immigrants and children.