There are 5 US Post Offices in Mccook County serving a total of 5,649 residents equating to 1,130 residents per post office. It's estimated that approximately 9,016 packages are handled across the 5 county post offices annually. McCook County is a county located in the U.S. state of South Dakota. At the 2010 census, the population was 5,618. Its county seat is Salem. The county was established in 1873 and organized in 1878. It was named for the former governor of the Dakota Territory and Civil War general Edwin Stanton McCook. McCook County is part of the Sioux Falls, SD Metropolitan Statistical Area.