There are 3 US Post Offices in Campbell County serving a total of 848 residents equating to 283 residents per post office. It's estimated that approximately 1,354 packages are handled across the 3 county post offices annually. Campbell County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 1,466, making it the fifth-least populous county in South Dakota. Its county seat is Mound City. The county was created in 1873 and organized in 1884. It is named after Norman B. Campbell, a Dakota Territory legislator in 1873 and son of General Charles T. Campbell."