There are 10 US Post Offices in Washington County serving a total of 13,872 residents equating to 1,387 residents per post office. It's estimated that approximately 22,141 packages are handled across the 10 county post offices annually. Washington County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 14,716. Its county seat is Nashville. It is located in the southern portion of Illinois known locally as \"Little Egypt\"."