Cass County is a county located in the western part of the U.S. state of Missouri. As of the 2010 census, the population was 99,478. Its county seat is Harrisonville. The county was organized in 1835 as Van Buren County but was renamed in 1849 after U.S. Senator Lewis Cass of Michigan who later became a presidential candidate.Cass County is part of the Kansas City, MO Metropolitan Statistical Area.