A county in Oregon is defined as a corporate or quasi-corporate body established for the purpose of local self-government; an agent of the state; and a territorial and political division of the state. A county exercises powers granted and defined by the Oregon Constitution, the Oregon Legislative Assembly, referendum, and the Oregon Supreme Court.
A British and American agreement for joint occupancy of the northwest region of the North American continent was signed in 1818. What came to be called Oregon Country was claimed by both the United States and Great Britain until 1846 when a treaty set the boundary between the two countries at 49° north latitude.
There was no organized movement among the Americans to establish a local government until 1841 when meetings were held at the Methodist Mission at Willamette Falls on the Willamette River. A constitution was drawn up and offices were created; however, the government did not prove successful.
A provisional government, with a set of laws and offices, was established in 1843 in a series of meetings at Champoeg along the Willamette River. The provisional government was replaced by a territorial government when Oregon became a U.S. territory in 1848. The territorial government lasted until 1859 when Oregon became the 33rd state admitted to the Union. Under the provisional and territorial governments the county was established as the unit of local government and county offices were created.
Oregon Country was divided into four districts in 1843: Champoeg, Clackamas, Tuality, and Yamhill. Champoeg extended from the Willamette River east to the summit of the Rocky Mountains and from the California border to the 45th parallel just south of the Columbia River. Clackamas extended from the site of present day Portland north to the 54° 40' parallel and east to the summit of the Rocky Mountains and included the present state of Washington, northern Idaho, and portions of Montana and Canada. Tuality extended from the Portland area north to the 54° 40' parallel and west to the Pacific Ocean. Yamhill extended from the Willamette River west to the Pacific Ocean and south to the California border.
In 1844 a fifth district, Clatsop, was taken from the western half of Tuality; and the northern boundary of Oregon, specifically Clatsop, Tuality, and Clackamas Counties, was established at the Columbia River.
When the Oregon Territory was created in 1848 it
included the present state of Washington. In 1853 Washington Territory was formed from the northern part of the Oregon Territory and the eastern boundary of Oregon was established along the Snake River and south to the Nevada border. The borders of the state of Oregon have not changed since 1853.
With the creation of Clatsop district in 1844, Oregon began the long process of forming and reforming, naming and renaming counties. The last county to be created was Deschutes in 1917.