Sullivan County is part of the Kingsport–Bristol (TN)–Bristol (VA) Metropolitan Statistical Area, which is a component of the Johnson City–Kingsport–Bristol, TN-VA Combined Statistical Area – commonly known as the "Tri-Cities" region.
As of the census of 2000, there were 153,048 people, 63,556 households, and 44,806 families residing in the county. The population density was 371 people per square mile (143/km²). There were 69,052 housing units at an average density of 167 per square mile (65/km²). The racial makeup of the county was 96.55% White, 1.89% Black or African American, 0.22% Native American, 0.43% Asian, 0.01% Pacific Islander, 0.21% from other races, and 0.69% from two or more races. 0.71% of the population were Hispanic or Latino of any race.
There were 63,556 households out of which 28.40% had children under the age of 18 living with them, 57.10% were married couples living together, 10.20% had a female householder with no husband present, and 29.50% were non-families.according to the United States Census Bureau. Of 63,556 households, 1,915 are unmarried partner households: 1,702 heterosexual, 97 same-sex male, 116 same-sex female. 26.40% of all households were made up of individuals and 11.10% had someone living alone who was 65 years of age or older. The average household size was 2.36 and the average family size was 2.84.
In the county, the population was spread out with 21.80% under the age of 18, 7.30% from 18 to 24, 28.40% from 25 to 44, 26.50% from 45 to 64, and 15.90% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 93.30 males. For every 100 females age 18 and over, there were 90.20 males.
The median income for a household in the county was $33,529, and the median income for a family was $41,025. Males had a median income of $31,204 versus $21,653 for females. The per capita income for the county was $19,202. About 9.70% of families and 12.90% of the population were below the poverty line, including 17.10% of those under age 18 and 11.90% of those age 65 or over.