Mandan is a city on the eastern border of Morton County, North Dakota, United States. Founded in 1879 on the west side of the upper Missouri River, it was designated in 1881 as the county seat of Morton County. The population was 24,206 at the 2020 census, and was estimated at 24,788 in 2024, making it the eighth-most populous city in North Dakota. Across the Missouri River from Bismarck. Mandan is a core city of the Bismarck metropolitan area.