According to the United States Census Bureau, the town has a total area of 37.1 square miles (96.0 km2), of which 34.7 square miles (89.9 km2) is land and 2.4 square miles (6.1 km2), or 6.37%, is water.
There were 4,646 households, out of which 31.3% had children under the age of 18 living with them, 50.3% were headed by married couples living together, 12.5% had a female householder with no husband present, and 30.6% were non-families. 23.3% of all households were made up of individuals, and 7.4% were someone living alone who was 65 years of age or older. The average household size was 2.57 and the average family size was 3.00.
In the town the population was spread out with 22.9% under the age of 18, 8.4% from 18 to 24, 27.2% from 25 to 44, 30.8% from 45 to 64, and 10.7% who were 65 years of age or older. The median age was 39.6 years. For every 100 females there were 99.8 males. For every 100 females age 18 and over, there were 97.8 males.
At the 2000 census, the median income for a household in the town was $40,156, and the median income for a family was $48,852. Males had a median income of $57,869 versus $17,441 for females. The per capita income for the town was $21,196. About 6.1% of families and 9.2% of the population were below the poverty line, including 6.1% of those under age 18 and 7.6% of those age 65 or over.
Voter Registration and Party Enrollment as of October 25, 2005
John Cantius Garand (1888 – 1974), Canadian-American designer of firearms best known for creating the first successful semi-automatic rifle to be widely used in active military service, the M1 Garand. The Interstate 395 bridge that crosses the Quinebaug River on the Griswold/Lisbon town line is named in his honor.