European settlement of Jefferson County began in the late 1700’s. It was organized in 1818 and named in honor of former President Thomas Jefferson. Archeologists have found remains of eight different Indian cultures dating back 10,000 years. Christian Wilt and John Honey laid out new Hartford, the first town in the county, in 1806, near present day Herculaneum. Along with seven other counties, Jefferson County was formed from parts of Saint Louis and Ste. Genevieve Counties by an “Act of the Territory” December 8, 1818. It was named “Jefferson” in honor of the third President of the United States and father of the Louisiana Purchase. Herculaneum, with a population of 200, was named the first county seat in 1821, the same year Missouri attained statehood. By 1831, a more central location for a county seat was desired. At the west edge of Hillsboro flowed a spring. A good water source and the fact that Hillsboro was a stopping point in the trail that passed between Potosi and St. Louis became deciding factors in relocating the county seat. The railroad shops of De Soto, the discovery of high quality sand in Crystal City, the establishment of the largest lead smelter in the United States, and the entry of the Frisco Railroad added greatly to the county's continued growth.

Dry goods merchant Theodore Kimms, who named the town after him, founded Kimmswick in 1859. He laid the town out on 160 acres on land purchased from the widow of Captain George Waters. German immigrant stonecutters and wealthy St. Louisans settled the area. There are numerous salt springs in the area, which were used by Native Americans as a source of salt. These salt springs also attracted prehistoric animals; nearby is the Mastodon State Historic Site, where bones of this extinct elephant were discovered. A spa and resort area in Kimmswick was a popular steamboat daytrip destination for Saint Louisians in the late 19th century. The Kimmswick Historical Society, founded in 1969, was instrumental in saving the town and preserving its historic architecture. The town is now a popular tourist destination, and features numerous antique and craft stores and country-style restaurants. Goldman is a tiny ghost town in Jefferson County about 35 miles south of St. Louis, Missouri and five miles north of the county courthouse at Hillsboro, Missouri. Goldman is located on Old Lemay Ferry Road, an old trade route connecting Hillsboro and St. Louis. Goldman is adjacent to the historic Sandy Creek Covered Bridge, the only remaining covered bridge in eastern Missouri and a state historic site. Mapaville is located about halfway between Festus and Hillsboro on Missouri Supplemental Route A. This area is believed to be the home of infamous bandit Jake Dallas, of the Cole Box gang, which robbed stage coaches and trains. The gang is best known for their gunfights with Sheriff Kyle Proemsey and his deputies including the hot-tempered Ryan "Quick Draw" Murphy. Dallas and the Cole Box gang were eventually gunned down by Proemsey and Quick Draw. Arnold was incorporated in 1972, and is named after George Arnold, a local landowner. The first European settler in Arnold was Jean Baptiste Gamache, who operated a ferry boat across the Meramec River in exchange for 1050 arpents of land granted by the King of Spain. This ferry was on the King's Trace or El Camino Real, (royal road), from St. Louis, Missouri, to Ste. Genevieve, Missouri. This road became known as Telegraph after 1850, after the first telegraph lines in the state were put along it. Hillsboro is the county seat of Jefferson County. Hillsboro is the English equivalent of the Italian Monticello, home of President Thomas Jefferson. The founders of Jefferson County had hoped to use the name of the home of their county's namesake for the county seat, but that honor in Missouri had already been taken by Monticello in Lewis County.

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