There are 1 US Post Offices in Stanley County serving a total of 2,835 residents equating to 2,835 residents per post office. It's estimated that approximately 4,525 packages are handled across the 1 county post offices annually. Stanley County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 2,966. Its county seat is Fort Pierre. The county was created in 1873 and organized in 1890. Stanley County is included in the Pierre, SD Micropolitan Statistical Area."