What is now Jefferson County was included in Lewis County, created by the Oregon Provisional Government on December 19, 1845. Lewis County included all of the "Oregon Country" north of the Columbia River, west of the Cowlitz River and the Cascade Mountains, and north to Latitude 54°-40', the southern boundary of the Russian claims in Alaska.

Oregon Territory was created by the U.S. Congress in 1848. The Oregon Territorial Legislature created Jefferson County (including modern day Clallam County) in 1852 and appointed Henry C. Wilson as the first Sheriff in 1853. In 1854 Jefferson County assumed its current boundaries when the Oregon Territorial Legislature removed the northern half to create Clallam County.

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