IDesign Cafe provided our Walk Through Temperature/Metal detectors to the Jefferson County Sheriff and Jefferson County. Jefferson County is located in the eastern portion of the state of Missouri. It is a part of the St. Louis Metropolitan Area. 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.