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 spread over 12,000 square
 miles (31,000 km2), 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.