Cahokia Courthouse is one of the region's oldest buildings and a unique remnant of the French occupation of the mid-Mississippi Valley. Originally constructed in about 1740 as a dwelling, the building became a courthouse in 1790. For twenty-four years it served as a center of judicial and political activity in the old Northwest Territory.
The post-on-sill, or poteaux-sur-solle, structure consists of upright hewn logs seated in a horizontal sill log, and is an excellent example of French colonial-era timber construction. Dismantled in 1901 and exhibited at the St. Louis World's Fair, it was moved again in 1906 to Chicago. The courthouse was finally returned to its original site as a W.P.A. project. The historic building has housed a public museum since its dedication in 1940 to the "splendid heritage" of the citizens of Illinois.
After officials moved the St. Clair County seat to Belleville in 1814, the building was converted to use as a town hall, a public school, a home again, and even a saloon for a time. By the 1890s the former courthouse served to store farm machinery and appeared on the verge of collapse.
The court at Cahokia had jurisdiction over a vast territory reaching, at one time, to the Canadian border. This court, which met on a quarterly basis, heard all cases
of a civil or criminal nature. It settled disputes, provided for orphans and the poor, and dispensed justice swiftly.