Johnson County is named for Colonel Richard Mentor Johnson and not Andrew Johnson, as some believe. Colonel Johnson was in the War of 1812 and the vice-president of the U.S. during the Van Buren administration. 

Until the late 1830s the area known today as Johnson County was nothing more than wilderness and a hunting ground for the Fox and Sac Indians. Between 1832 and 1837 the Indians were stripped of their lands by treaty. Soon settlers began to move into the county. The county was established in 1837. By 1838 there were two towns competing for the county seat--Osceola and Napoleon. An Act of the Wisconsin Assembly selected Napoleon as the county seat on June 22, 1838. A two-story log cabin courthouse was then constructed at Napoleon. 

When the First Legislative Assembly of the Territory of Iowa met at Burlington a bill was approved to locate the seat of government for the Territory of Iowa. The site was to be within the boundaries of Johnson County, and it was to be named Iowa City. Three commissioners met and selected a site two miles north of Napoleon on May 1, 1839. 

On December 31, 1839, an Act was passed to relocate the county seat of Johnson County with orders to keep present and future populations in mind. The site of the new county seat was easily chosen as Iowa City, the "capital" of the Iowa Territory. In January 1855, the Fifth General Assembly voted to move the state capital to Des Moines. 

The first courthouse built in Iowa City was a temporary two-story structure constructed of bricks. This courthouse burned to the ground in 1856. In 1857 this building was replaced by another courthouse, which was insured for $30,000. This brick building was used until 1899, when it was condemned. 

The cornerstone of the present courthouse was laid on December 2, 1899. It was constructed by James Rowson and Son at a cost of $135,000. Bands, balloons and a parade were part of the dedication ceremonies held on June 8, 1901. 

Johnson County is currently composed of 11 cities: Iowa City, Coralville, Lone Tree, Tiffin, Oxford, North Liberty, Shueyville, University Heights, Hills, Swisher, Solon and a subdivision of West Branch. Johnson County is home to more than 100,000 Iowans and a wide variety of businesses, including farming, retail, manufacturing and academics.

The county government's responsibilities range from roads and bridges to human services, law enforcement, parks and natural resources. Iowa City is the seat of Johnson County and, as such, many of the county government offices are located there.

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In 2011, Johnson County, Iowa had a total population of 133,000 - 67,000 (50 percent) females and 66,000 (50 percent) males. The median age was 29.3 years. Nineteen percent of the population was under 18 years and 9 percent was 65 years and older.

For people reporting one race alone, 87 percent were White; 4 percent were Black or African American; less than 0.5 percent were American Indian and Alaska Native; 6 percent were Asian; less than 0.5 percent were Native Hawaiian and Other Pacific Islander, and 2 percent were Some other race. Three percent reported Two or more races. Five percent of the people in Johnson County, Iowa were Hispanic. Eighty-three percent of the people in Johnson County, Iowa were White non-Hispanic. People of Hispanic origin may be of any race.