For geographic and demographic information on the census-designated place Williamstown, please see the article Williamstown (CDP), Massachusetts.

Williamstown is the northwesternmost town in Massachusetts; that is, it shares its northern border with Vermont and its western border with New York.

There are 2,753 households out of which 24.7% have children under the age of 18 living with them, 51.3% are married couples living together, 7.9% have a woman whose husband does not live with her, and 38.5% are non-families. 32.7% of all households are made up of individuals and 15.7% have someone living alone who is 65 years of age or older. The average household size is 2.21 and the average family size is 2.80.

In the town the population is spread out with 15.3% under the age of 18, 27.5% from 18 to 24, 16.6% from 25 to 44, 21.0% from 45 to 64, and 19.6% who are 65 years of age or older. The median age is 36 years. For every 100 females there are 87.8 males. For every 100 females age 18 and over, there are 85.8 males.

The median income for a household in the town is $51,875, and the median income for a family is $67,589. Males have a median income of $50,011 versus $32,845 for females. The per capita income for the town is $26,039. 5.5% of the population and 1.7% of families are below the poverty line. Out of the total people living in poverty, 1.3% are under the age of 18 and 6.8% are 65 or older.