[6][7] This was part of the large territory of the Osage people, who occupied lands up to present-day St. Louis.

[8] 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 the period of Bleeding Kansas, prior to the residents voting on whether slavery would be allowed in the territory.

In 1862, during the Civil War, Confederate guerrillas from nearby Missouri, led by William Quantrill, raided the Johnson County communities of Olathe and Spring Hill.

Developer J. C. Nichols spurred the boom in 1914 when he built the Mission Hills Country Club to attract upscale residents who previously had been reluctant to move from Missouri to Kansas.

Integration of public schools in Kansas City, Missouri, resulted in many white families leaving the inner city, resulting in increased migration to the county for new housing and what were considered higher quality public schools, generally an indicator of higher economic status.

Kill Creek begins in the southwestern portion of the county and flows northward into the Kansas River at De Soto.

Mill Creek begins in the central portion of the county in Olathe, flowing northward it empties into the Kansas River at Shawnee.

Brush Creek flows east-northeastward through Prairie Village and Mission Hills, entering Kansas City, Missouri, within the median of Ward Parkway and passing the Country Club Plaza before emptying into the Blue River east of the Country Club Plaza and north of Swope Park.

Each flows northeastward meeting in Leawood, where the stream retains the name of Indian Creek, just before crossing the state line and entering the Blue River in Kansas City, Missouri.

The Blue River begins in rural southern Johnson County and flows north-northeastward through the southeastern portion of the county and crossing the state line just east of the intersection of 151st Street and Kenneth Road in southern Overland Park.

Bull Creek and Little Bull Creek begin in rural southwestern Johnson County and flow southward where they enter Hillsdale Lake before continuing into Franklin County, eventually joining the Marais des Cygnes at Paola.

The county consists primarily of prairie grassland, with corridors of forested areas along streams and rivers.

[20] The county voted "No" on the 2022 Kansas abortion referendum, an anti-abortion ballot measure, by 69% to 31%, outpacing its support of Joe Biden during the 2020 presidential election.

[21][22] Johnson County is entirely located within Kansas's 3rd congressional district, which has been represented by Democrat Sharice Davids since 2019.

The county even denied Franklin Roosevelt in all four of his campaigns, with his closest margin coming in 1932, where he lost by just two votes.

[24][25] However, in 2016, Republican Donald Trump won the county by less than three points, as the GOP’s shift toward right-wing populism alienated moderate voters.

This leftward shift was mirrored in the state’s Senate races, where Democrats won Johnson County in 2020 and 2022, despite losing statewide.

Once a solid Republican base, the county now leans Democratic at the national level, reflecting broader trends in Kansas’s suburban areas.

In both the 2022 and 2024 elections, Democrats expanded their majority on the Board and flipped the Sheriff’s office for the first time in 96 years.

[28][29] Numerous Republicans from the area identify as moderates, compared to some of the more ideological hard-liners from other parts of the state.

Differences between moderates and the more hard-line members can most commonly be seen on social issues, the most infamous being the numerous debates about the state's school finance formula in 2004[30] and 2014–2018.

For instance, decisions regarding the regulation of land use, development and zoning in unincorporated areas of the county are the responsibility of the county government, whereas such decisions for areas within incorporated places are the jurisdiction of the incorporated city of which the property is a part.

[47] In April 2024, Johnson County Sheriff Calvin Hayden claimed that he "had a search warrant in hand" when local election officials "decided in a hurry to destroy" ballots from 2019, 2020 and 2021, despite Kansas state law ordering the regular destruction of old ballots, and the local officials having informed Hayden in November 2023 that they would move to destroy the old ballots, which Hayden had asked to be preserved during his investigation of an election software firm.

The northern section is older, while the southern portion is the fastest-growing area in Johnson County, containing a massive volume of new homes.

Johnson County was originally divided into nine townships, two of which have since been eliminated by the annexation of all their territory into independent municipalities.