Johnson County was established in 1855 and officially organized on September 7, 1857. One of the first 33 counties in the state, it was named for the Reverend Thomas Johnson, founder of the Shawnee Methodist Mission.
By the time Johnson County was organized, a number of communities already were established within its boundaries. The first was Olathe Town company, incorporated on February 20, 1857. Soon thereafter, the towns of Spring Hill, Gardner, De Soto, Gum Springs (present day Shawnee) also were incorporated.
In 1870, there were 13,000 people residing in the county. The population remained relatively stable and the economy agriculturally based until the 1910s when the northeastern section of the county began to develop. Lured by J.C. Nichols' innovatively designed communities, Mission Hills in the early 1910s and the Country Club District in the early 1920s, the population began to grow.
In 1904, W. B. Strang began construction of an interurban electric railroad between Kansas City and Olathe. It passed through his newly planned community, Overland Park, and the town of Lenexa. A second interurban railroad, the Hocker Grove Line, was constructed south and west through Merriam and Shawnee.
In the decades following World War II, the county's population exploded. By 1950, the number of residents grew to 63,000, from 18,288 in 1910. Since 1950, transportation corridors such as Interstates 35 and 435, which opened rural areas to new development, have stimulated population and building growth.
By 2000, the county's population has reached 443,000 and projections indicate that it will exceed 630,000 by 2020. Current growth and development patterns are to the south, southwest, and western areas of the county.