Williamsville is located mostly within the town of Amherst, but Creek Road and Creek Heights in the south part of the village are in the town of Cheektowaga. The village is in the northeastern quadrant of Erie County.
Williamsville is also a postal ZIP code: 14221 (14231 for PO Boxes); however, that zip code contains large sections of the town outside of the village and also includes significant commercial areas within Clarence.
The Williamsville School District is a school system covering Williamsville, most of the eastern part of Amherst, and a small portion of the western end of Clarence.
As of the census of 2000, there were 5,573 people, 2,534 households, and 1,337 families residing in the village. The population density was 4,447.8 people per square mile (1,721.4/km²). There were 2,640 housing units at an average density of 2,107.0/sq mi (815.4/km²). The racial makeup of the village was 97.29% White, 0.68% African American, 0.11% Native American, 0.95% Asian, 0.32% from other races, and 0.65% from two or more races. Hispanic or Latino of any race were 1.08% of the population.
There were 2,534 households out of which 22.6% had children under the age of 18 living with them, 42.1% were married couples living together, 8.9% had a female householder with no husband present, and 47.2% were non-families. 42.3% of all households were made up of individuals and 21.7% had someone living alone who was 65 years of age or older. The average household size was 2.11 and the average family size was 2.93.
In the village, the population was spread out with 19.4% under the age of 18, 5.2% from 18 to 24, 26.6% from 25 to 44, 22.9% from 45 to 64, and 25.8% who were 65 years of age or older. The median age was 44 years. For every 100 females there were 75.7 males. For every 100 females age 18 and over, there were 71.3 males.
The median income for a household in the village was $47,557, and the median income for a family was $65,300. Males had a median income of $43,500 versus $32,172 for females. The per capita income for the village was $27,177. About 2.5% of families and 4.5% of the population were below the poverty line, including 2.7% of those under age 18 and 4.7% of those age 65 or over. It is one of the wealthiest suburbs of the Buffalo metropolitan area.