Nassau County had a population of 1,356,924 at the 2019 American Community Survey. This was an increase of 17,392 since the 2010 census. At the 2010 U.S. Census, there were 1,339,532 people in the county, with 448,527 households and 340,523 families. The U.S. Census Bureau estimated Nassau County's population to have grown by 2.2% to 1,369,514 in 2017.
In 2010, the county had 340,523 family households, out of which 33.5% had children under the age of 18. About 60% of the population were married couples living together, 11.7% had a female householder with no spouse present, and around 24.1% were non-families. 15.1% of the households had someone living alone and aged 65 years or older, with 24.1% of the households made up of individuals. The average family size was 3.38, and the average household size was 2.94. Nassau County's population was 18.7% aged 62 years or older and 23.3% under the age of 18. The median age was 41.1 years. For every 100 females in Nassau County, there were 93.7 males. For every 100 females above 18 years, there were 90.4 males.
Houses for sale in Nassau County, New York, are greatly valued given the high median household income. For example, at the 2019 American Community Survey, the county had a median household income of $116,100. The per capita income was $51,422. However, about 3.6% of the population in Nassau County lived below the poverty line. In 2010, the average household income in Nassau County was 72,030. The average family income was $81,246, rising significantly from the figures recorded in 2007. The average income for males stood at $52,340, with females having $37,446. The county had a per capita income of $32,151. About 2.8% of the families and 3.6% of the population lived below the poverty line. Out of this figure, 5.8% were under the age of 18, while 5.6% were aged over 65 years.