Jefferson County is a county located in the eastern portion of the state of Missouri. As of the 2010 census, the population was 218,733, making it the sixth-most populous county in Missouri. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former President Thomas Jefferson. In 1980 the county contained the mean center of U.S. population. Jefferson County is part of the St. Louis, MO-IL Metropolitan Statistical Area and encompasses many of the city’s southern suburbs. Governor Mel Carnahan was killed near Goldman, Missouri in a plane crash on October 16, 2000.
