Leavenworth, Kansas. Leavenworth is the largest city in and the county seat of Leavenworth County, Kansas, United States. As of the 2010 census, the city population was 35,251. Located on the west bank of the Missouri River 25 mi (40 km) northwest of Kansas City, Missouri, it is part of the Kansas City metropolitan area. The site of Fort Leavenworth, built in 1827, the city became known in American history for its role as a key supply base in the settlement of the American West.