There are 9 US Post Offices in Washington County serving a total of 22,388 residents equating to 2,488 residents per post office. It's estimated that approximately 35,734 packages are handled across the 9 county post offices annually. Washington County is a county located in the eastern portion of the U.S. state of Missouri. As of the 2010 census, the population was 25,195. The largest city and county seat is Potosi. The county was officially organized on August 21, 1813, and was named in honor of George Washington, the first President of the United States."