Most of the city lies within Jackson County, but portions spill into Clay, Cass, and Platte counties. The northwest quadrant contains Wyandotte, and Leavenworth, counties in Kansas and parts of Platte County, Missouri. Kansas City Metropolitan Statistical Area (MSA) · Bates County, Mo. The origin of modern-day Kansas City, Missouri dates back to the 1830s, when John McCoy founded the settlement of Westport at what is now Westport Road. PLEASE BE ADVISED: Jackson County Historic Truman Courthouse Closed September 10-20, 2020.