Clarendon County is a county located below the fall line in the Coastal Plain region of U.S. state of South Carolina. As of the 2020 census, its population was 31,144. Its county seat is Manning. This area was developed for lumber and mills, including textile mills. Clarendon County boasts one of the largest man-made lakes in the United States, Lake Marion, completed in 1941 as a New Deal project. It was planned as part of a national rural electrification initiative.