Clark County was created as Vancouver County from portions of Clackamas and Tuality Counties. In 1849, the legislative assembly renamed the area Clark County. The county was named for Captain William Clark of the Lewis and Clark Expedition, and is located in the southwest comer of Washington, just north of Portland, Oregon.

General Joseph Lane, first governor of Oregon Territory, appointed William Ryan sheriff of Clark County on Sept. 8, 1849. At the same time David Stone was appointed prosecuting attorney for the Third Judicial District, which contained Clark, Clatsop, and Lewis Counties. The first election for county officers was held July 1, 1850, even though the laws establishing county boards of commissioners and their functions was not passed until Jan. 20, 1851.

The newly organized probate court selected a site for the county seat of Clark County, and authorized R. H. Landsdale to survey the town site. At the time Vancouver was called Columbia City. On Jan. 26, 1855 the county seat's name was changed from Columbia City to Vancouver.

Clark County was the earliest settled area in western Washington. Fort Vancouver was established in 1824 and the regional headquarters of the Hudson's Bay Company was located near the fort in 1825. The Hudson's Bay Company exerted great influence in the area until the signing of the US Great Britain Boundary Treaty (1846). In 1849, Fort Vancouver became a permanent US Army post.

In 1852 the Monticello Convention adopted a memorial asking Congress to create a new "Territory of Columbia." Congress approved the bill to establish the new territory but changed the name to Washington in March, 1853. With the creation of Washington Territory on March 2, 1853, Clark County was no longer under the jurisdiction of Oregon Territory.

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