Thornwood is a hamlet and census-designated place (CDP) in the town of Mount Pleasant in Westchester County, New York. The population was 3,759 at the 2010 census.
As of the census of 2000, there were 5,980 people, 1,886 households, and 1,559 families residing in the hamlet. The population density was 1,760.9 per square mile (679.1/km²). There were 1,921 housing units at an average density of 565.7/sq mi (218.1/km²). The racial makeup of the hamlet was 90.97% White, 2.36% African American, 0.03% Native American, 4.00% Asian, 0.07% Pacific Islander, 1.39% from other races, and 1.19% from two or more races. Hispanic or Latino of any race were 5.80% of the population.
There were 1,886 households out of which 40.7% had children under the age of 18 living with them, 72.5% were married couples living together, 7.2% had a female householder with no husband present, and 17.3% were non-families. 14.8% of all households were made up of individuals and 6.9% had someone living alone who was 65 years of age or older. The average household size was 3.06 and the average family size was 3.39.
In the hamlet the population was spread out with 28.3% under the age of 18, 6.3% from 18 to 24, 28.3% from 25 to 44, 23.9% from 45 to 64, and 13.3% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 97.8 males. For every 100 females age 18 and over, there were 95.7 males.
The median income for a household in the hamlet was $88,971, and the median income for a family was $98,313. Males had a median income of $73,438 versus $40,365 for females. The per capita income for the hamlet was $36,254. About 2.2% of families and 4.0% of the population were below the poverty line, including 2.6% of those under age 18 and 3.9% of those age 65 or over.
Thornwood is the home to the North American headquarters for the Carl Zeiss Corporation, a multinational German manufacturer of optical systems, industrial measurements and medical devices.