The Provisional Legislature created Washington County as Tuality District on July 5, 1843, as one of the four original counties created in Oregon. Tuality County was bordered on the north by latitude 54deg. 40min., on the east by Clackamas County, on the west by the Pacific Ocean, and on the south by Yamhill County. In 1844, the Columbia River was made the northern boundary of Tuality County and Clatsop County was created from Tuality County's western half. In 1849, the Territorial Legislature changed the name of Tuality County to Washington in honor of President George Washington. Washington County obtained its present boundaries in 1854 with the creation of Columbia County to the north and Multnomah County to the east. The area of Washington County is 727 square miles. 

Columbia was selected as the county seat in 1850. Later, the town was given the name of Hillsborough, which in 1858 was modified to Hillsboro. After meeting in two local church buildings, a permanent county headquarters was built in 1852. A brick courthouse was built in 1873 and remodeled in 1891. In 1912 an annex was built. A new courthouse was built in 1928 to replace the original brick structure. The Justice Services Building was added on to the courthouse in 1972. The Public Services Building was constructed in 1990 to house most county offices. The Walnut Street Center opened in 1999 and houses Land Use and Transportation Department offices. 

Although Tuality County government was established in 1843, a complete county government was not organized until 1845 when three county judges having probate and administrative authority, a treasurer, and a sheriff were appointed. The first election of county and territorial officers was held in 1846. 

The voters of Washington County approved a home rule charter at the general election in 1962. The new county government was headed by the county administrative officer and board of county commissioners, which is the five-member policy making body of the county. Under home rule, most county services are provided by broad functional departments, such as assessment and taxation, health and human services, and land use and transportation. 

Washington County has been one of the fastest developing areas in Oregon in recent decades. The 1990s saw a growth spurt of 42.94%. The 2011 population of 536,370 marked an increase of over 20% since 2000. The development of a large electronics industry during the last two decades has contributed significantly to the economy of the county. Other principle industries are agriculture, manufacturing, and food processing. The headquarters of Nike, the world's leading supplier of athletic shoes, apparel, and sports equipment, are near Beaverton.