Marion County, originally named Champooick District (later Champoeg), was created July 5, 1843, by the Provisional Legislature. Champoeg District stretched south to the California border and east to the Rocky Mountains. The area, however, was soon reduced with the creation of Wasco, Linn, Polk, and other counties. Marion County's present geographical boundaries, established in 1856, are the Willamette River and Butte Creek on the north, the Cascade Range on the east, the Santiam River and North Fork of the Santiam on the south, and the Willamette River on the west.
The county shares political borders with Clackamas, Yamhill, Polk, and Linn Counties. It contains 1,194 square miles. Champoeg District was redesignated a county in 1845 and renamed Marion County in 1849 after General Francis Marion, a Revolutionary War hero. That same year Salem was designated the county seat. The territorial capital was moved from Oregon City to Salem in 1852. The ensuing controversy over the location of the capital was settled in 1864 when Salem was confirmed as the state capital.
Salem has had three county courthouses, all located on the same site. The first courthouse was completed in 1854 and replaced in 1873. During the 1930s and 1940s efforts to move the 1873 building and preserve it as a museum failed and it was demolished in 1952. The third and present courthouse was completed in 1954. Arson fires in 2005 caused severe damage to the courthouse, causing a long closure before renovations were completed. The Courthouse Square Building next to the courthouse was constructed in 2000 and houses many Marion County administrative offices.
Marion County had a county court form of government for the first century of its existence. The county court exercised a combination of executive, administrative, and judicial functions; however as the judicial branch of state government developed, the necessity for the county court to exercise judicial functions diminished. In 1941 the county was divested of all judicial responsibilities, and the remaining vestiges of probate and civil jurisdiction were transferred to the circuit court. In 1961 the Legislative Assembly enabled a county court with no judicial functions to reorganize as a board of county commissioners. With court approval, the Marion County Court was abolished and became the Marion County Board of Commissioners in 1963.
The 2011 population of 318,150 represented an increase of 0.9% over 2010.
The county is located in the center of the Willamette Valley. Agriculture and food processing are important to the county's economy, as are lumber, manufacturing, and education. Government is the county's main employer and economic base.