There are 5 US Post Offices in Washington County serving a total of 18,756 residents equating to 3,751 residents per post office. It's estimated that approximately 29,937 packages are handled across the 5 county post offices annually. Washington County is a county in the U.S. state of Nebraska. As of the 2010 census, the population was 20,234. Its county seat is Blair. Washington County is part of the Omaha-Council Bluffs, NE-IA Metropolitan Statistical Area. In the Nebraska license plate system, Washington County is represented by the prefix 29 (it had the 29th-largest number of vehicles registered in the county when the license plate system was established in 1922)."