Cass County is a county in the U.S. state of Nebraska. As of the 2020 United States Census, the population was 26,598. Its county seat and largest city is Plattsmouth. The county was formed in 1855, and was named for General Lewis Cass, an American military officer, politician, and statesman. Cass County is included in the Omaha–Council Bluffs, NE–IA Metropolitan Statistical Area. In the Nebraska license plate system, Cass County is represented by the prefix 20.