Johnson County (county code JO) is a county located in the U.S. state of Kansas. As of the 2010 census, the county population was 544,179, making it the most populous county in Kansas. Its county seat is Olathe, and its most populous city is Overland Park.
Johnson County is included in the Kansas City, MO-KS Metropolitan Statistical Area. The county contains many of Kansas City's affluent southwestern suburbs. The county has the highest median household income and highest per-capita income in Kansas and is among the most affluent in the United States, with the 19th highest median household income in 2000 and the 46th highest per-capita income in 2005. In 2010, Money magazine, in its list of the 100 Best Cities in the United States in which to live, ranked Overland Park 7th (ranked 6th in 2006 and 9th in 2008) and Shawnee 17th (ranked 39th in 2008). In 2008 the same magazine also ranked Olathe 11th.
Johnson County is named for missionary Thomas Johnson, and was one of the first counties established in the Kansas Territory in 1855. The Oregon-California and Santa Fe Trails, which pass through nearby Independence, Missouri, also passed through the county. The renowned gunfighter Wild Bill Hickok settled for a time in the county, becoming constable of Monticello Township in 1858. Johnson County was the site of many battles between abolitionists and pro-slavery advocates during Bleeding Kansas. In 1862 Confederate Guerrillas from nearby Missouri led by William Quantrill raided the Johnson County communities of Olathe and Spring Hill, killing half a dozen men and destroying numerous homes and businesses.
The county was largely rural until the early 20th century, when housing subdivisions were developed in the northeastern portion of the county adjacent to Kansas City, Missouri. Developer J.C. Nichols spurred the boom in 1914 when he built the Mission Hills Country Club to lure upscale residents who previously had been reluctant to move from Missouri to Kansas. Suburban development continued at a steady pace until the close of World War II. Following the war, the pace of development exploded, triggered by the return of soldiers in need of housing and white flight, a common occurrence throughout the United States during the post-war period. The US Supreme Court 1954 school desegregation ruling and its impact in Kansas City, Missouri contributed to white flight throughout the region, resulting in increased migration from Kansas City, Missouri to the county and the furthering the county's reputation as a destination for quality public schools and as a good location for families. This growth slowed slightly in the 1970s, however beginning in the mid-1980s the pace of growth increased significantly, with the county adding 100,000 residents each decade between the 1990 census and 2010 census.