As of the census2 of 2000, there were 91,025 people, 33,800 households, and 24,099 families residing in the county. The population density was 59/km² (153/mi²). There were 38,605 housing units at an average density of 25/km² (65/mi²). The racial makeup of the county was 71.13% White, 22.50% Black or African American, 0.87% Native American, 0.65% Asian, 0.07% Pacific Islander, 3.21% from other races, and 1.57% from two or more races. 5.86% of the population were Hispanic or Latino of any race.

There were 33,800 households out of which 36.00% had children under the age of 18 living with them, 53.20% were married couples living together, 13.50% had a female householder with no husband present, and 28.70% were non-families. 23.30% of all households were made up of individuals and 8.50% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.07

The median income for a household in the county was $35,105, and the median income for a family was $41,176. Males had a median income of $30,265 versus $22,283 for females. The per capita income for the county was $16,775. 14.90% of the population and 11.30% of families were below the poverty line. 17.20% of those under the age of 18 and 19.40% of those 65 and older were living below the poverty line.

The county is divided into thirteen townships: Anderson Creek, Averasboro, Barbecue, Black River, Buckhorn, Duke, Grove, Hectors Creek, Johnsonville, Lillington, Neills Creek, Stewarts Creek, and Upper Little River. The cities and towns of Harnett County are Angier, Broadway, Buies Creek, Coats, Dunn, Erwin, and Lillington. Neighboring areas include Wake County, Johnston County, Sampson County, Cumberland County, Moore County, Lee County, and Chatham County.