most populous county in kansas
Johnson County, Kansas. 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. In the mid-19th century, this was part of the Shawnee Reservation after their removal from east of the Mississippi River.