There are 3 US Post Offices in Washington County serving a total of 10,165 residents equating to 3,388 residents per post office. It's estimated that approximately 16,224 packages are handled across the 3 county post offices annually. Washington County is a county in the U.S. state of Idaho. As of the 2010 census, the population was 10,198. The county seat and largest city is Weiser, with over half of the county's population. The county was established in 1879 when Idaho was a territory and named after U.S. president George Washington. Washington County is home to the Idaho ground squirrel."