St. Mary's Church, built in 1817, is the oldest church still in use in Rockville. Rockville was chosen for the church location for its relatively large concentration of Catholics, it central location, and its prominence as the County seat. St. Mary's became the church from which other upcounty mission churches sprang.
Father James Redmond established an eight-member congregation here in 1816. A member of the congregation sold four acres of land to the church for $300. Parishioners helped construct the Georgian-style building, which was the first brick Catholic Church in the County. The bricks arches were hand-rubbed to create a contrasting color and texture to the brick walls. The main building was 53 feet long by 36 feet wide. A 15-foot square extension at the rear included a small sacristy and the pastor's living quarters. Father Redmond offered the first Mass in St. Mary's Church on December 14, 1817.
Although the church has been remodeled and enlarged several times, most of the original church remains today and seats approximately 150. In the 1960s, a new church was built next to it, and a demolition permit was issued for the old church in 1966. It was saved at the last moment by parishioners and concerned citizens. The American writer, F. Scott Fitzgerald, and his wife Zelda, and members of his family are buried in the adjacent cemetery.