Yamhill County was one of the first 'Districts' of Oregon, created on July 5, 1843. At that time it encompassed an area lying between the Willamette River and the Pacific Coast and extending north-south from the Columbia River to the California border.
Lying on the west side of the lower middle part of the Willamette Valley, Yamhill County is bounded by Washington, Clackamas, Polk, Marion and Tillamook Counties.