Mount Pleasant is a large suburban town in Charleston County, South Carolina, United States. In the Lowcountry, it is the fourth-most populous municipality in South Carolina, and for several years was one of the state's fastest-growing areas, doubling in population between 1990 and 2000. The population was 90,801 at the 2020 census. It is part of the Charl…Mount Pleasant is a large suburban town in Charleston County, South Carolina, United States. In the Lowcountry, it is the fourth-most populous municipality in South Carolina, and for several years was one of the state's fastest-growing areas, doubling in population between 1990 and 2000. The population was 90,801 at the 2020 census. It is part of the Charleston metropolitan area.