Yamhill County is a county located in the Willamette Valley region of the U.S. state of Oregon. According to Oregon Geographic Names, the origin of the name is uncertain, but is probably from an explorer's name for a local Native American tribe, the Yamhill, who are part of the North Kalapuyan family. The population of Yamhill County was 99,193 in the 2010 census. The county seat is located at McMinnville.
The earliest known inhabitants of the area were the Yamhill Indians, who have inhabited the area for over 8000 years. They are one of the tribes incorporated into the Confederated Tribes of the Grand Ronde. In 1857 they were forced to migrate to the Grand Ronde Indian Reservation created in Oregon's Coastal Range two years earlier.
The earliest non-native settlers were employees of the various fur companies operating in Oregon Country, who started settling there around 1814. But it was the establishment of the Oregon Trail that led to significant migration to the area.
Yamhill District (later county) was created on July 5, 1843, five years before the Oregon Territory was established. It was one of the original four districts created by Oregon's first Provisional Legislature, along with Twality (later Washington), Clackamas, and Champooick (later Marion) counties. The district was originally over , an area that was broken up into twelve present-day counties.
Lafayette, the principal trading center of the western Willamette Valley in early Oregon history, was made the county seat in 1847. The county government was later (1889) moved to McMinnville where it remains today.
The Mount Hebo Air Force Station was a Cold War air defense installation from 1956 to 1980. Located next to Tillamook County, at the top of high Mount Hebo, Air Force radars operated by the 689th Radar Squadron and the 14th Missile Warning Squadron were essential parts of the nation's integrated air defenses. The large radomes protecting the radars from adverse weather effects could be seen silhouetted against the sky from many parts of Yamhill County.