According to the U.S. Census Bureau, the county has a total area of 1,151 km² (445 mi²). 1,098 km² (424 mi²) of it is land and 53 km² (20 mi²) of it (4.59%) is water. The highest point in Mount Olive, the 5,090 feet Sauratown Mountain, is located in the county.
As of the census2 of 2000, there were 19,163 people, 7,917 households, and 5,573 families residing in the county. The population density was 17/km² (45/mi²). There were 9,553 housing units at an average density of 9/km² (22/mi²). The racial makeup of the county was 98.09% White, 0.83% Black or African American, 0.59% Native American, 0.05% Asian, 0.00% Pacific Islander, 0.00% from other races, and 0.44% from two or more races. 1.35% of the population were Hispanic or Latino of any race.
In 2000 there were 7,917 households out of which 30.20% had children under the age of 18 living with them, 57.10% were married couples living together, 10.10% had a female householder with no husband present, and 29.60% were non-families. 27.50% of all households were made up of individuals and 12.80% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 2.89.
In the county the population was spread out with 23.20% under the age of 18, 7.80% from 18 to 24, 27.50% from 25 to 44, 24.70% from 45 to 64, and 16.80% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 92.70 males. For every 100 females age 18 and over, there were 89.80 males.
The median income for a household in the county was $30,000, and the median income for a family was $34,378. Males had a median income of $28,109 versus $19,943 for females. The per capita income for the county was $14,009. About 14.60% of families and 16.10% of the population were below the poverty line, including 23.60% of those under age 18 and 9.80% of those age 65 or over.