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 18,331 at the 2010 census. Mandan was founded in 1879 and became the county seat in 1881. Mandan is located just across the Missouri River from Bismarck. According to the United States Census Bureau, the city has a total area of 10.3 square miles (27km2). 10.2 square miles (26km2) of it is land and 0.1 square miles (0.26km2) of it (1.07%) is water.