There are 5 US Post Offices in Marshall County serving a total of 4,324 residents equating to 865 residents per post office. It's estimated that approximately 6,902 packages are handled across the 5 county post offices annually. Marshall County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 4,656. Its county seat is Britton. The county is named after Marshall Vincent, who homesteaded near Andover, South Dakota."