Yamhill was the second of the 4 original districts created by the Provisional Legislature in 1843. Its boundaries were drawn to include the area from the Willamette River west to the Pacific Ocean and from the Yamhill River south to the California border. The district consisted of 12,000 square miles; however, 12 counties were eventually created from Yamhill County leaving 709 square miles within its present borders. The county shares borders with Washington County to the north, Tillamook County to the west, Polk County to the south, and Marion and Clackamas Counties to the east.
The county was named for the original inhabitants of the area, the Yamhill Indians, a tribe of the Kalapooian family, who lived around the Yamhill River. The tribe was moved to the Grand Ronde Reservation in 1855. The earliest non-native settlers entered the area in 1814; most were employees of the fur companies operating in Oregon. Many immigrants who came over the Oregon Trail during 1843-1844 settled in the Yamhill region, which became the agricultural center of the Willamette Valley.
Lafayette, at one time the principal trading center of the western Willamette Valley, became the county seat in 1847. The first courthouse, purchased in 1850, was originally a county store in Lafayette. The building was destroyed by fire in January 1857, and all records except probate and land records were destroyed. The next courthouse was built in 1858 and remained in use until the county seat moved in 1889 to McMinnville where a new courthouse was built. The fourth and present courthouse was built in 1964.
Yamhill County government originally consisted of three commissioners, district attorney, assessor, clerk, sheriff, surveyor, and treasurer. In 1964 the probate function was transferred from the jurisdiction of the county court to the district court. The county court was abolished in 1968 and the board of commissioners was established in 1969.
Yamhill County ranks high among Oregon counties in annual market value of its agricultural production. Today, the county's primary industry is agriculture, specifically wheat, barley, horticulture, and dairy farming. Yamhill County is also the center of Oregon's wine industry, which drives an important tourism business. About 1/3 of the county is covered with commercial timber, and an economic mainstay of the western part of the county is logging and timber products. Non-seasonal light industries have also located in Yamhill County. Much of the county's workforce commutes to the Portland metropolitan area.
The population of Yamhill County in 2016 was 104,990, which represented a 5.8% increase over 2010.