Simpson County is a county located in the U.S. state of Mississippi. Its western border is formed by the Pearl River, an important transportation route in the 19th century. As of the 2020 census, the population was 25,949. The county seat is Mendenhall. The county is named for Josiah Simpson, a territorial judge who also served as a delegate to Mississippi's Constitutional Convention. Simpson County is part of the Jackson, MS Metropolitan Statistical Area.