Mastic Beach is a census-designated place and former village in the southeast part of the town of Brookhaven in Suffolk County, New York, United States. The population was 12,930 at the 2010 census, at which time it was an unincorporated census-designated place for the first time. The village is located in the general vicinity of the hamlet of Mastic Beach.
According to the United States Census Bureau, the CDP had a total area of 5.3 square miles (14 km), of which 4.2 square miles (11 km) was land and 1.1 square miles (2.8 km), or 20.34%, was water.
As of the census of 2000, there were 11,543 people, 3,755 households, and 2,793 families residing in the CDP. The population density was 2,727.9 per square mile (1,053.6/km²). There were 4,375 housing units at an average density of 1,033.9/sq mi (399.3/km²). The racial makeup of the CDP was 88.16% White, 5.02% African American, 0.49% Native American, 0.91% Asian, 0.01% Pacific Islander, 2.64% from other races, and 2.77% from two or more races. Hispanic or Latino of any race were 10.59% of the population.
There were 3,755 households out of which 42.6% had children under the age of 18 living with them, 51.3% were married couples living together, 16.1% had a female householder with no husband present, and 25.6% were non-families. 18.4% of all households were made up of individuals and 6.3% had someone living alone who was 65 years of age or older. The average household size was 3.07 and the average family size was 3.49.
In the CDP, the population was spread out with 31.8% under the age of 18, 8.5% from 18 to 24, 32.4% from 25 to 44, 19.7% from 45 to 64, and 7.5% who were 65 years of age or older. The median age was 32 years. For every 100 females, there were 99.8 males. For every 100 females age 18 and over, there were 96.4 males.
The median income for a household in the CDP was $44,937, and the median income for a family was $49,219. Males had a median income of $37,871 versus $27,853 for females. The per capita income for the CDP was $17,046. About 9.2% of families and 11.3% of the population were below the poverty line, including 14.3% of those under age 18 and 8.1% of those age 65 or over.