Cass County is a county in the U.S. state of North Dakota. As of the 2020 census, the population was 184,525. Cass County is the most populous county in North Dakota, accounting for nearly a quarter of the state's population. The county seat is Fargo, the state's most populous city. The county is named for George Washington Cass, president of the Northern Pacific Railway from 1872 to 1875. It is the only Cass County in the United States that is not named after Lewis Cass.