Washington County, one of Oregon's 36 counties, is the most populous county with a population of almost 600,000 people! The largest city in Washington County is also the county seat, Hillsboro. The median household income of Washington County is over $83,000 and the median property value is almost $420,000.
The original boundaries included the entire northwest corner of Oregon before sections became new counties. The Tualatin River and its drainage basin are almost entirely within the county, with the county nearly coterminous with the Tualat in Valley. The county has a total area of 726 square miles where 724 square miles is land and 2.2 square miles is water. The county's highest point is South Saddle Mountain at 3,464 feet above sea level.
Originally named Twality when created in 1843, the territorial legislature renamed it for the nation's first president in 1849. Twality was one of the original four districts of the Provisional Government of Oregon. Columbia, later known as Hillsboro, was selected as the county seat in 1850.