There are 7 US Post Offices in Washington County serving a total of 24,236 residents equating to 3,462 residents per post office. It's estimated that approximately 38,683 packages are handled across the 7 county post offices annually. Washington County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 21,704. The county seat is Washington. Washington County is included in the Iowa, IA Metropolitan Statistical Area."