Clackamas County, named after the Clackamas Indians, was one of the four original districts created by the Provisional Legislature on July 5, 1843. In 1843 Clackamas County covered portions of four present-day states and one Canadian province. The Columbia River was made the northern boundary of the county in 1844. The United States-Great Britain Boundary Treaty of 1846 relocated the northern border of both the United States and Clackamas County at latitude 49deg. The Act of Congress that created Washington Territory in 1853 enclosed Clackamas County within the present-day boundaries of Oregon. Clackamas County acquired its current boundaries in 1854. The county is bounded by Multnomah County to the north, Wasco County to the east, Marion County to the south, and Yamhill and Washington Counties to the west. The county encompasses 1,879 square miles.

Oregon City became the county seat for Clackamas County. The city was built on a portion of Dr. John McLoughlin's land claim. In 1844 Oregon City was incorporated by the Provisional Legislature, making it the first incorporated city west of the Rocky Mountains. This area was the terminus for water transportation on the Willamette River and had been a meeting place for Indians, hunters, trappers, and Hudson's Bay Company voyagers for years.

The first major overland immigration to Oregon City occurred in 1842. Three years later Samuel Barlow established an immigrant route that ran from The Dalles, around the south side of Mt. Hood, and into Oregon City. The Barlow Road funneled thousands of immigrants into Oregon City and Clackamas County during the 1840s. Oregon City rapidly became the primary urban center in Clackamas County and dominated social and political life in Oregon during the provisional government period. The removal of the territorial capital from Oregon City to Salem in 1852 shifted most of the political activity to Salem. The creation of Multnomah County in 1854, at the request of Portland residents, removed Oregon's principal commercial city from Clackamas County as well as the county's access to the Columbia River.

Prior to construction of a courthouse, county records were housed in several locations in Oregon City, including the former provisional state house. Two of the locations burned but the county records were saved. In 1884 a frame and concrete structure was built at a cost of $145,000. In 1935 the county records were moved temporarily to rented quarters while a new courthouse was built. Using construction grants available through the Works Progress Administration the current courthouse was completed in 1937.

Clackamas County government is composed of three commissioners, a district attorney, assessor, clerk, sheriff, surveyor, and treasurer.

Heavily timbered, the county's geographical features include numerous rivers - the Willamette, Clackamas, Sandy, Pudding, Molalla, and Salmon, and Mt. Hood. Since its creation, agriculture, timber, manufacturing, and commerce have been the county's principal activities. The population of Clackamas County has steadily increased from 1850 through 1997. The 2000 population of 338,391 represented a 21.35% increase over 1990.