There are 6 US Post Offices in Clark County serving a total of 3,664 residents equating to 611 residents per post office. It's estimated that approximately 5,848 packages are handled across the 6 county post offices annually. Clark County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 3,691. Its county seat is Clark. The county was created in 1873 and organized in 1881. It was named for Newton Clark, a South Dakota legislator in 1873."