St. Mary's is the oldest of the three Roman Catholic parishes in Charlestown, established early in the second quarter of the nineteenth century, and its original 1828 church, on a different site, was only the second Catholic church constructed within the immediate Boston area. The church is a late example of the work of Patrick C. Keely, the best-known and most prolific designer of nineteenth-century Roman Catholic churches in the Northeast. The Gothic Revival nave and corner tower church is constructed of Rockport granite with brick trim. King-post trusses distinguish the open nave interior with figures supporting the paneled, molded, and traceried wood ceiling. Characteristic of large Roman Catholic parishes, St. Mary's, though more dispersed than most, includes a complex of buildings to accommodate the parishioners' religious, educational, and social needs. Across Winthrop Street stands the former St. Mary's Parochial School, designed by Keely's successor firm, Keely and Houghton, in 1901–1902. In the mid-1980s the school was adapted for elderly housing with several multistory additions.