After passage of the Kansas-Nebraska Act, May 30, 1864, this area was swiftly settled. The county was organized in 1855, taking its name from Gen. Henry Leavenworth who established the nearby fort in 1827.
In a county seat war, Leavenworth was victor over its rivals Kickapoo and Delaware in 1857. At first county offices were located in the city hall. The first courthouse building, completed on this site in 1873, was destroyed by fire in 1911. The replacement structure, built in 1912, was remodeled in 1951.