Mandan is a city in Morton County, North Dakota in the United States. It is the county seat of Morton County. It is a core city of the Bismarck-Mandan Metropolitan Statistical Area. The population was 16,718 at the 2000 census. Mandan was founded in 1879 and became the county seat in 1881. Mandan is located just across the Missouri River from Bismarck. Mandan is home to the North Dakota Association of Rural Electric Cooperatives