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.