Benton County was created on December 23, 1847 by an act of the Provisional Government of Oregon. At the time of its formation the county included all the country west of the Willamette River, south of Polk County and running all the way to the California border in the south and the Pacific Ocean in the west.
The county was created out of lands originally inhabited by the Klickitat, who rented it from the Kalapuyas for use as hunting grounds. All aboriginal claims to land within Benton County were ceded in the Treaty of Dayton in 1855. Portions of Benton County were taken to form Coos, Curry, Douglas, Jackson, Josephine, Lane and Lincoln Counties, leaving Benton County in its present form.
The original governing body of Benton County was the Probate Court comprised of three elected judges. Shortly after Oregon was granted territorial status the County Court system was adopted and comprised of two elected commissioners and the elected County Judge. Many Oregon counties have maintained the County Court system to the current day.
Since 1972, Benton County has been an Oregon "Home Rule" County, meaning that the citizens have full control over the county charter, rather than using a standard constitution issued by the state. The voters elect three County Commissioners, a District Attorney and a Sheriff to four-year terms.