Jefferson County’s 200,000 residents comprise the largest rural population in the state of Missouri. To the north is St. Louis County, the Mississippi River is on the east side, and to the south and west are Ste. Genevieve County, St. Francois County, Washington County and Franklin County.

The county was fashioned from parts of Saint Louis and SainteGenevieve counties on Dec. 8, 1818. It was named after Thomas Jefferson, the third President of the United States and father of the Louisiana Purchase. Herculaneum was the first county seat, but by 1831, Hillsboro was selected because it was near a spring, and was a stopping point along the trail that passed between Potosi and St. Louis. Some of its cities and towns:
 

Among the many points of interest are several archaeological sites on the National Register of Historic Places. Archeologists have found remains of eight different Indian cultures dating back 10,000 years in the area. European settlement of Jefferson County began in the late 1700s. Around 1774, John Hildebrand, from France, set down roots near Saline Creek in the northeastern part of the county, an area later known as the Meramec settlement. Other county sites of historical significance include the De Soto Historical Society Museum; Fenton History Museum; Fletcher House (built by Missouri’s first native-born governor); Mastodon State Historic Site (where bones of American mastodons have been found) and the Missouri Pacific Historical Society Museum.

In recent years, Jefferson County has planned to build a casino Barnhart, on a 150-foot bluff overlooking the Mississippi River. When completed, the casino is expected to employ more than 1,000 people, making it the largest employer in Jefferson County.