As of the census of 2000, there were 19,972 people, 7,708 households, and 5,449 families residing in the county. The population density was 47 people per square mile (18/km²). There were 10,548 housing units at an average density of 25 per square mile (10/km²). The racial makeup of the county was 38.90% White, 54.49% Black or African American, 4.79% Native American, 0.13% Asian, 0.03% Pacific Islander, 0.79% from other races, and 0.88% from two or more races. 1.59% of the population were Hispanic or Latino of any race.
There were 7,708 households out of which 28.20% had children under the age of 18 living with them, 49.20% were married couples living together, 17.30% had a female householder with no husband present, and 29.30% were non-families. 26.20% of all households were made up of individuals and 12.20% had someone living alone who was 65 years of age or older. The average household size was 2.48 and the average family size was 2.97.
In the county the population was spread out with 23.50% under the age of 18, 8.00% from 18 to 24, 26.30% from 25 to 44, 24.80% from 45 to 64, and 17.40% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 95.00 males.
The median income for a household in the county was $28,351, and the median income for a family was $33,602. Males had a median income of $26,928 versus $20,787 for females. The per capita income for the county was $14,716. About 15.70% of families and 19.40% of the population were below the poverty line, including 24.90% of those under age 18 and 20.80% of those age 65 or over.
Warren County is heavily populated by descendants of a long existing tri-racial isolate deeply rooted in the area.