[2] Sullivan County is included in the Claremont-Lebanon, NH-VT Micropolitan Statistical Area.

It is named for John Sullivan (1740–1795), the Revolutionary War hero and a former governor.

At the 2000 census,[9] there were 40,458 people, 16,530 households and 11,174 families living in the county.

The racial makeup of the county was 97.99% White, 0.24% Black or African American, 0.29% Native American, 0.37% Asian, 0.02% Pacific Islander, 0.14% from other races, and 0.94% from two or more races.

There were 16,530 households, of which 29.40% had children under the age of 18 living with them, 54.70% were married couples living together, 8.60% had a female householder with no husband present, and 32.40% were non-families.

25.70% of all households were made up of individuals, and 10.90% had someone living alone who was 65 years of age or older.

As of the 2010 United States census, there were 43,742 people, 18,126 households, and 12,025 families living in the county.

There were 22,341 housing units at an average density of 41.6 per square mile (16.1/km2).

[10] In terms of ancestry, 23.9% were English, 16.3% were Irish, 9.2% were German, 7.1% were Italian, 6.0% were French Canadian, 5.2% were Scottish, 5.1% were Polish, and 4.4% were American.