There are 12 US Post Offices in Washington County serving a total of 57,576 residents equating to 4,798 residents per post office. It's estimated that approximately 91,898 packages are handled across the 12 county post offices annually. Washington County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 61,778. Its county seat is Marietta. The county, the oldest in the state, is named for George Washington. Washington County comprises the Marietta, OH Micropolitan Statistical Area, which is also included in the Parkersburg-Marietta-Vienna, WV-OH Combined Statistical Area."