Mastic is a hamlet and 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.
Mastic is served mostly by the William Floyd School District, but also the Eastport-South Manor Central School District north of Grove Drive in the Manor Park section of the hamlet.
The Poospatuck Indian Reservation lies entirely within the community, near its southern end and along the Forge River.
The northernmost section of the hamlet has been notably called Manor Park, which stretches from Sunrise Highway to Moriches-Middle Island Road immediately east of Brookhaven Airport. Part of the neighborhood lies within the hamlet of Shirley, but is served entirely by Mastic's zip code of 11950.
Mastic is located at 40°48′8″N 72°50′38″W (40.802090, -72.843953). According to the United States Census Bureau, the CDP has a total area of 4.0 square miles (10.3 km2), of which 3.9 square miles (10.1 km2) is land and 0.08 square miles (0.2 km2), or 2.13%, is water.
At the 2010 census there were 15,481 people, 4,526 households, and 3,743 families in the CDP. The population density was 3,969.5 per square mile (1,532.8/km2). There were 4,847 housing units at an average density of 1,242.8/sq mi (479.9/km2).