McElroy entered the Society, before its formal restoration, as a brother in 1806, though he was later ordained a priest. He served as procurator of the Maryland mission and worked at Georgetown College before becoming pastor of St. John's Church in Frederick in 1822. There, he collaborated with the Sisters of Charity to open a free school for girls, established a Jesuit-run St. John's Literary Institution for boys, facilitated the move of the province's novitiate across the street from both institutions, and built a new church, all while tending to the church's rural missions stretching from Maryland to Virginia. A famed retreat leader, McElroy traveled throughout the eastern United States leading religious communities and congregations in the Exercises. He also oversaw the construction of St. Joseph's Church in Philadelphia while still serving as Frederick's rector. In 1846, he received the appointment by the Polk administration as a de facto chaplain to the United States Army and was stationed in Matamoras during the country's war with Mexico. The other Jesuit appointed, Anthony Rey, during their service. McElroy returned north after a year and then labored for sixteen years to found Boston College and its adjacent Church of the Immaculate Conception. He secured the college's charter in 1863, on the eve of his eighty-first birthday and while he struggled with blindness. He died in 1877 at the novitiate in Frederick. He was reportedly the country's oldest priest and the world's oldest Jesuit at the time. 

See publications related to McElroy in the Jesuit Online Bibliography.