Johnson County is located in the U.S. state of Iowa. As of the 2020 census, the population was 152,854, and was estimated to be 160,044 in 2025, making it the fourth-most populous county in Iowa. The county seat and the largest city is Iowa City, home of the University of Iowa. Johnson County is included in the Iowa City metropolitan area, which is also included in the Cedar Rapids-Iowa City Corridor Combined Statistical Area.
History
Johnson County was established on December 21, 1837, by the legislature of the Wisconsin Territory, one of thirteen counties established by that body in a comprehensive act. The county's area was partitioned from Dubuque County and was not initially provided with a civil government, instead being governed by Cedar County officials. It was originally named for the United States Vice President Richard M. Johnson (1780–1850). In 2020, the Johnson County Board of Supervisors voted unanimously to change the county's namesake to be Lulu Merle Johnson (1907–1995), the first black woman in the state to earn a doctorate. Vice President Johnson had a common law enslaved wife (Julia Chinn) whom he could not marry since it was against the law at that time to have an interracial marriage. They had two children together and she was in charge of his plantation when he was away on business. He was open about his relationship with her, which was unusual for the time. Another motivation for the changing of the county namesake was Johnson's killing of numerous Native Americans during battles, including Tecumseh, and Johnson's enslavement of many people, including Chinn.
The first courthouse in the county was a two-story log cabin structure, built in 1838 in the settlement of Napoleon, about two miles south of the current courthouse. The building stood across from what later would become the James McCollister Farmstead on land later owned by Philip Clark.
thumb|left|Old Johnson County Courthouse, Iowa City, 1857-1899
After Iowa City was established by fiat as the new territorial capitol of Iowa, the county seat was removed there. The second Johnson County Courthouse, the first in Iowa City, was built on Lot 8 Block 8 of the County Seat Addition to Iowa City in 1842 for $3,690. It was built by James Trimble, who had previously built the first jail. The building was apparently built of brick with stone and wood ornamentation.
The Richardsonian Romanesque style courthouse in use today was designed by the firm of Rush, Bowman and Rush of Grand Rapids, Michigan. It was bid for $111,000 and built by the firm Rowson & Son of Johnson County. The cornerstone was laid in December 1899. The building's tower was based on Henry Hobson Richardson's design for the spire of Trinity Church in Boston. The building was dedicated on June 8, 1901. The currently unused jail that stands to the west of the courthouse was designed by C.L. Wundt of Burlington, Iowa, on behalf of the Stewart Iron Works in Cleveland with a bid of $14,000.
Geography
According to the United States Census Bureau, the county has a total area of , of which is land and (1.62%) is water. It is the 20th largest county in Iowa by total area.
Major highways
- x20px Interstate 80
- x20px Interstate 380
- x20px U.S. Highway 6
- x20px U.S. Highway 218
- x20px Iowa Highway 1
- x20px Iowa Highway 22
- x20px Iowa Highway 27
Transit
- 380 Express
- Cambus
- Coralville Transit
- Iowa City Transit
Adjacent counties
- Benton County – northwest
- Cedar County – east
- Iowa County – west
- Linn County – north
- Muscatine County – east and southeast
- Louisa County – southeast and south
- Washington County – south