As of the census of 2000, there were 18,324 people, 7,908 households, and 5,337 families residing in the county. The population density was 77 people per square mile (30/km²). There were 9,192 housing units at an average density of 39 per square mile (15/km²). The racial makeup of the county was 92.26% White, 5.89% Black or African American, 0.19% Native American, 0.24% Asian, 0.03% Pacific Islander, 0.63% from other races, and 0.76% from two or more races. 3.01% of the population were Hispanic or Latino of any race.
There were 7,908 households out of which 23.50% had children under the age of 18 living with them, 56.30% were married couples living together, 7.90% had a female householder with no husband present, and 32.50% were non-families. 28.90% of all households were made up of individuals and 15.00% had someone living alone who was 65 years of age or older. The average household size was 2.28 and the average family size was 2.78.
In the county the population was spread out with 20.10% under the age of 18, 5.80% from 18 to 24, 24.20% from 25 to 44, 26.30% from 45 to 64, and 23.60% who were 65 years of age or older. The median age was 45 years. For every 100 females there were 90.20 males. For every 100 females age 18 and over, there were 87.10 males.
The median income for a household in the county was $36,259, and the median income for a family was $45,096. Males had a median income of $29,375 versus $23,070 for females. The per capita income for the county was $19,804. 10.10% of the population and 6.40% of families were below the poverty line. 11.70% of those under the age of 18 and 8.80% of those 65 and older were living below the poverty line.
The interchange for I-26 and the U.S. 74 freeway is located in Columbus. Interstate 26 provides Polk County with easy access to the cities of Johnson City, Tennessee and Asheville, North Carolina to the north, and Spartanburg, South Carolina; Columbia, South Carolina; and the port of Charleston, South Carolina to the southeast. Interstate 26 junctions with the major Interstates of I-85 and I-40 are within a half-hour and forty minutes, respectively. East on U.S. 74 goes to Charlotte, North Carolina, and eventually to the port of Wilmington, North Carolina. Proximity to these highways places Polk County in a strategic position for potential business development in the future.
Polk County is also served by an additional non-freeway U.S. Highway: U.S. Route 176. This was the primary highway linking Saluda and Tryon to Hendersonville, North Carolina and Spartanburg, S.C. prior to the delayed completion of I-26 in 1976. Two North Carolina routes, N.C. 108 and N.C. 9, traverse the county as well. N.C. 108 begins in Rutherfordton, North Carolina and travels west through Columbus and ends at U.S. 176 in Tryon. Oriented north-to-south, N.C. 9 connects Black Mountain, North Carolina and Lake Lure, North Carolina to Spartanburg and points southeast via Polk County. N.C. 108 and N.C. 9 intersect at the unincorporated town of Mill Spring.
Polk County and Saluda are infamous among railroad enthusiasts for the Saluda Grade, the steepest standard-gauge mainline railway grade in the United States. Norfolk Southern suspended freight traffic indefinitely along this route in December 2001. The track remains in place, but are cut near Flat Rock, North Carolina and Landrum, South Carolina.