On September 3, 1849, the territorial legislature renamed it in honor of Francis Marion,[4] a Continental Army general from South Carolina who served in the American Revolutionary War.

Marion County was created by the Provisional Legislature of Oregon on July 5, 1843, as the Champooick District, one of the original four districts of the Oregon Country along with Twality (later Washington), Clackamas, and Yamhill counties.

Originally, this political entity stretched southward to the California border and eastward to the Rocky Mountains.

With the creation of Wasco, Linn, Polk, and other counties, its area was reduced in size.

Marion County's present geographical boundaries were established in 1856.

[5] As of the 2020 census, there were 345,920 people, the racial and ethnic makeup of the county was 61.8% non-Hispanic white, 1.1% African American, 0.9% Native American, 2.1% Asian, 4.9% of two or more races, and 27.7% Hispanic or Latino (of any race).

24.00% of all households were made up of individuals, and 9.50% had someone living alone who was 65 years of age or older.

Commissioners, who are full-time, salaried officials, have executive, legislative, and quasi-judicial powers (the latter in land-use cases).

The BOC is responsible for accepting funds from sources outside the county, strategic planning, and enacting ordinances as needed to carry out plans and serve the public.

The BOC also prepares a county budget in cooperation with the elected heads of the various departments.

County commissioners appoint and oversee non-elected department heads, officers, boards, and commissions.

In addition to the Board of Commissioners, the county has non-partisan positions that are elected in county-wide elections for four year terms: Assessor, Clerk, Treasurer, District Attorney, and Sheriff.

[a] Agriculture and food processing are important to the county's economy, as are lumber, manufacturing, and education.

Government, however, is the county's main employer and economic base.