Leavenworth County is located in the U.S. state of Kansas and is part of the Kansas City metropolitan area. Its county seat and most populous city is Leavenworth. As of the 2020 census, the county population was 81,881. The county was named after Henry Leavenworth, a general in the Indian Wars who established Fort Leavenworth.

The Kansas City metropolitan area is a bi-state metropolitan area anchored by Kansas City, Missouri. Its 14 counties straddle the border between the U.S. states of Missouri and Kansas. With 8,472 square miles (21,940 km2) and a population of more than 2.2 million people, it is the second-largest metropolitan area centered in Missouri and is the largest metropolitan area in Kansas, though Wichita is the largest metropolitan area centered in Kansas. Alongside Kansas City, Missouri, these are the suburbs with populations above 100,000: Overland Park, Kansas; Kansas City, Kansas; Olathe, Kansas; Independence, Missouri; and Lee's Summit, Missouri.