Mastic is a hamlet and a census-designated place (CDP) in the southeastern part of the town of Brookhaven in central Suffolk County, New York, United States. The population was 15,481 at the 2010 census. The hamlet was originally called Forge until 1893, when it was changed to the current name of Mastic. The Long Island Rail Road built a station in 1882 and, on July 15, 1960, the stop was moved 7,010 feet (2,140 m) west and renamed Mastic–Shirley. The Poospatuck Indian Reservation lies entirely within the community, near its southern end and along the Forge River.