McElroy is born on May 14, 1782 in Enniskillen, County Fermanagh, the younger of two sons. In the hopes of providing a better life for John and his brother Anthony, their father, a farmer, finances their travel to the United States. In 1803 the two young men board a ship leaving the port of Londonderry and arrive in Baltimore, Maryland, on August 26. McElroy eventually settles in Georgetown, Washington, D.C., and becomes a merchant.
In 1806, McElroy enters Georgetown College in Washington, D.C., the same year he enters the novitiate of the Society of Jesus as a lay brother. He eventually manages the finances of Georgetown College and in 1808 erects the tower building. He managed the school’s finances so well that through the period of economic hardship following the War of 1812, he is able to send several Jesuits to Rome to study.
McElroy is ordained in May 1817, after less than two years of preparation. As a new priest, he is assigned to Holy Trinity Catholic Church in Georgetown as an assistant pastor. In his short time at Trinity, he contributes to the growth of the congregation and enlarging of the church building. This is achieved by increasing the monthly subscription for congregation members from 12½ cents to $12.50 on July 3, 1819. The following day he travels to most of the congregation members’ homes and collects $2,000 in pledges. He immediately sets to work having the Church modified to include two lateral-wing chapels, which are first used on October 3, 1819.
On January 11, 1819, McElroy is granted United States citizenship. Also in 1819, McElroy starts a Sunday School for black children who are taught prayers and catechism simultaneously with spelling and reading, by volunteer members of the congregation. McElroy spends his remaining years in Georgetown teaching the lower grades.
In 1823, McElroy begins negotiations with the Sisters of Charity in Emmitsburg, Maryland, for the establishment of a school for girls in Frederick. In 1824, the St. John’s Benevolent Female Free School is founded by the Sisters of Charity of Saint Joseph at 200 East Second Street in Frederick. In 1825, McElroy sets to work replacing the pre-American Revolution log cabin that houses the school with a modern building large enough to also house an orphanage.
McElroy’s next task is to found an educational institution for boys. On August 7, 1828, the construction of St. John’s Literary Institute begins. The following year the construction is completed and the school is opened, a school which is currently operating under the name of Saint John’s Catholic Prep.
In October 1847, McElroy is welcomed in Boston, Massachusetts, by the Bishop of Boston, John Bernard Fitzpatrick, to serve as pastor of St. Mary’s parish in the North End. Bishop Fitzpatrick sets McElroy to work on bringing a college to Boston.
In 1853, McElroy finds a property in the South End where the city jail once stood. After two years of negotiations the project falls through due to zoning issues. A new site is identified and city officials endorse the sale. In 1858, Bishop Fitzpatrick and Father McElroy break ground for Boston College, and for the Church of the Immaculate Conception. Classes began in the fall of 1864, and continue at this location until 1913 when the college moves to its current location at Chestnut Hill. Initially Boston College offers a 7-year program including both high school and college. This joint program continues until 1927 when the high school is separately incorporated.
In 1868, McElroy retires to the Jesuit novitiate in Frederick, Maryland. He visits Georgetown for the final time in 1872 to celebrate his golden jubilee. His eyesight is failing and while moving through his home he falls, fracturing his femur, which eventually leads to his death. Father John McElroy dies September 12, 1877 at the Jesuit novitiate in Frederick, Maryland. For some years leading up to his death, he is regarded as the oldest priest in the United States and the oldest Jesuit in the world. He is buried in the Novitiate Cemetery. In 1903, the Jesuits withdraw from Frederick and the graves are moved from the Frederick Jesuit Novitiate Cemetery to St. John’s Cemetery.