Major roads include sections of Interstate 5 and 205, the Sunset Highway, Oregon Route 217, 47, 10, 6 and 8.

The Provisional Legislature of Oregon created the county as Twality District on July 5, 1843.

Nike successfully lobbied the legislature for a law that would prohibit their annexation for 99 years.

The Portland Metro Urban Growth Boundary (UGB) bisects the county.

The county's highest point is South Saddle Mountain at 3,464 feet (1,056 m) above sea level in the Northern Oregon Coast Range.

[9] Most of the county is in the Tualatin Valley, formed by the Tualatin Mountains to the east and north, the Chehalem Mountains to the south, and the Northern Oregon Coast Range to the west and north.

[20] Like all of the Yankee-influenced Willamette Valley and Oregon Coast, Washington County was in its pre-Depression history strongly Republican.

[21] In 1932, Franklin D. Roosevelt became the first-ever Democrat to win the Washington County vote, and he repeated this success in 1936 and 1940.

Between 1944 and 1988, Washington County was never won by a Democrat except in Lyndon B. Johnson's landslide victory in 1964.

[22] Gerald Ford's 17,529-vote margin of victory in the county was decisive in narrowly carrying the state of Oregon during that year's presidential election.

No Republican presidential candidate has carried Washington County since George H. W. Bush did so in 1988, and in four of the past five presidential elections, Barack Obama, Hillary Clinton, Joe Biden, and Kamala Harris all prevailed the county by over twenty percentage points.

The last Republican to win a statewide election in Washington County was Gordon H. Smith in the 2002 Senate contest.

Washington County is centered on a fertile plain that attracted farmers before the first wagon trains.

[citation needed] Agriculture is still a major industry in Washington County, as are lumber, manufacturing, and food processing.

The development of a large electronics industry during the 1980s and 1990s is the dominating factor of the county economy, and contributing to the creation of Oregon's Silicon Forest.

[27] Nike, one of two Fortune 500 corporations based in Oregon, has its headquarters in Washington County.

Other companies with headquarters in Washington County include optical instruments manufacturer Leupold & Stevens, Columbia Sportswear, and Reser's Fine Foods.