Cass County is a county located in the U.S. state of North Dakota. As of the 2010 census, the population was 149,778, and was estimated to be 167,005 in 2014. Cass County is the most populous county in North Dakota, accounting for nearly 23% of the population. It is the 14th-fastest growing county in the United States. Its county seat is Fargo, the most populous city in the state.