The county was formed in 1855 from parts of Henderson County and Rutherford County. It was named for William Polk, a colonel in the American Revolutionary War.
According to the U.S. Census Bureau, the county has a total area of 239 square miles (620 km2), of which, 238 square miles (620 km2) of it is land and 1 square mile (2.6 km2) of it is water. The total area is 0.31% water.
The county's largest body of water is Lake Adger, located about 5 miles (8.0 km) north of Columbus.
The elevation in the county ranges from just under 800 feet (240 m) near the confluence of the Green River and Broad River to over 3,200 feet (980 m) on Tryon Peak and Wildcat Spur, the highest peak in the county. Polk County is divided into two physiographic regions; the Blue Ridge Mountains in the western third of the county and Piedmont for the eastern two-thirds. Since it is in a transition zone between the two regions, Polk County is often referred to as being in the foothills.
The county is divided into six townships: Columbus, Cooper's Gap, Green Creek, Saluda, Tryon, and White Oak.