Jefferson County is a county located in east central Missouri in the United States. The county was included as the mean center of U.S. population in 1980. It is the sixth most-populous county in Missouri. As of the 2010 U.S. Census, the county’s population was 218,733. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former President Thomas Jefferson.
Jefferson County is part of the St. Louis Metro Area and consists of many of the city’s southern suburbs.