There are 6 US Post Offices in Harrison County serving a total of 7,580 residents equating to 1,263 residents per post office. It's estimated that approximately 12,099 packages are handled across the 6 county post offices annually. Warrants Harrison County is a county located in the northwest portion of the U.S. state of Missouri. As of the 2010 census, the population was 8,957. Its county seat is Bethany. The county was organized February 14, 1845, and named for U.S. Representative Albert G. Harrison of Missouri.