Clay County, South Dakota: History
The future Clay County area was opened for legal settlement in 1859. In Autumn 1859, Ahira A. Partridge (who would become the first elected sheriff of the county) crossed the Missouri river into the Dakota territory, and became the first white man to settle, on 160 acres of land that now underlies Vermillion. In 1862 the county was formally organized. The Clay County Courthouse was built in 1912.
Clay County is the name of 17 other counties in the United States, most of them named for Henry Clay.