Saline County, Illinois, was officially established on February 25, 1847, when Governor Augustus C. French approved an act introduced by the State Legislature to divide Gallatin County. As a result, Saline County was formed from the western half of Gallatin County, becoming the 99th out of the current 102 counties in Illinois. Initially, Raleigh was selected as the county seat, but in 1859, Harrisburg assumed this role.

Shortly after its formation, an effort was made to dissolve Saline County. In response, the three members of the Saline County Board retained a young lawyer, Abraham Lincoln, to represent their interests. The case ultimately reached the Illinois Supreme Court, where Lincoln successfully argued, ensuring that Saline County remained intact.