There are 10 US Post Offices in Pennington County serving a total of 55,637 residents equating to 5,564 residents per post office. It's estimated that approximately 88,803 packages are handled across the 10 county post offices annually. Pennington County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 100,948, making it the second-most populous county in South Dakota. Its county seat is Rapid City. The county was created in 1875 and organized in 1877. It is named for John L. Pennington, fifth Governor of Dakota Territory, who held office in 1875 when the county was formed. Pennington County is included in the Rapid City, SD Metropolitan Statistical Area. It is also the location of the Mount Rushmore National Memorial."