Millstone was incorporated as a borough by an Act of the New Jersey Legislature on May 14, 1894, from portions of Hillsborough Township, based on the results of a referendum held that day. The borough was reincorporated on March 12, 1928.
Added in 1976, Millstone Borough is listed in the National Register of Historic Places and encompasses 58 buildings. The borough possesses a military significance for 1700-1749, 1750-1799, 1850-1874.
New Jersey Monthly magazine ranked Millstone as its 7th best place to live in its 2008 rankings of the "Best Places To Live" in New Jersey.
As of the census of 2000, there were 410 people, 169 households, and 126 families residing in the borough. The population density was 547.1 people per square mile (211.1/km). There were 173 housing units at an average density of 230.9/sq mi (89.1/km). The racial makeup of the borough was 97.56% White, 0.98% African American, 0.98% Asian, and 0.49% from two or more races. Hispanic or Latino of any race were 3.17% of the population.
There were 169 households out of which 25.4% had children under the age of 18 living with them, 59.8% were married couples living together, 9.5% had a female householder with no husband present, and 24.9% were non-families. 18.9% of all households were made up of individuals and 8.9% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.79.
In the borough the population was spread out with 19.3% under the age of 18, 4.1% from 18 to 24, 25.4% from 25 to 44, 34.1% from 45 to 64, and 17.1% who were 65 years of age or older. The median age was 46 years. For every 100 females there were 99.0 males. For every 100 females age 18 and over, there were 91.3 males.
The median income for a household in the borough was $76,353, and the median income for a family was $83,118. Males had a median income of $60,156 versus $36,406 for females. The per capita income for the borough was $30,694. About 3.1% of families and 4.6% of the population were below the poverty line, including 6.4% of those under age 18 and 7.3% of those age 65 or over.