Note: The town of Clifton Forge is not included in any Alleghany County population data in the main article. In 2000, Clifton Forge was an independent city separate from the county. However, in 2001, Clifton Forge relinquished its city charter and reincorporated as a town; as a town, it is now a part of Alleghany County. The 2000 population of what is now Alleghany County (including Clifton Forge) was 17,215. The article includes geographic data from before and after the reincorporation of Clifton Forge into the county.
As of the census of 2000, there were 12,926 people, 5,149 households, and 3,866 families residing in the county. The population density was 29 people per square mile (11/km²). There were 5,812 housing units at an average density of 13 per square mile (5/km²). The racial makeup of the county was 96.35% White, 2.45% Black or African American, 0.21% Native American, 0.24% Asian, 0.02% Pacific Islander, 0.20% from other races, and 0.53% from two or more races. 0.36% of the population were Hispanic or Latino of any race. 42.9% were of American, 11.6% German, 11.0% English and 9.8% Irish ancestry according to Census 2000.
There were 5,149 households out of which 29.90% had children under the age of 18 living with them, 63.20% were married couples living together, 8.10% had a female householder with no husband present, and 24.90% were non-families. 22.20% of all households were made up of individuals and 10.50% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 2.85.
The age distribution is 22.80% under the age of 18, 6.20% from 18 to 24, 26.80% from 25 to 44, 28.50% from 45 to 64, and 15.70% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 99.60 males. For every 100 females age 18 and over, there were 95.30 males.
The median income for a household in the county was $38,545, and the median income for a family was $45,843. Males had a median income of $35,120 versus $20,855 for females. The per capita income for the county was $19,635. About 4.90% of families and 7.10% of the population were below the poverty line, including 8.60% of those under age 18 and 10.80% of those age 65 or over.