The old courthouse was constructed first as a dwelling about 1730 and around 1740, the dwelling was turned into a courthouse. It was the center of political activities in the Old Northwest Territory for more than 20 years.
In 1901 the courthouse was dismantled and shipped for storage in East St. Louis, Illinois. During the 1904 World's Fair, it was reconstructed and dismantled once again. After the fair, the old Cahokia Courthouse was reconstructed twice more in a smaller version in Chicago. Cahokians demanded the return of the courthouse in the 1920's and in 1936 the courthouse was reconstructed in its present location.
The Cahokia Courthouse is an excellent example of early French log construction known as post-on-sill foundation. The logs are seated upright on a horizontal sill log. The spaces between the logs are silled with stone and mortar chinking. The courthouse presently rests on its original foundation nearly 2 feet thick. Walnut beams extend the cantilever roof over the porch. The building consists of four rooms; a courtroom, a schoolroom, and offices for attorneys and clerks.
For twenty-four years the Cahokia courthouse served as a United states territorial courthouse. During this time Illinois Country passed through two reorganizations, becoming part of the Indiana Territory in 1800 and Illinois Territory in 1809. When St. Clair County was enlarged in 1801, territorial Governor William Henry Harrison made Cahokia Courthouse the judicial and administrative center of a vast area extending to the Canada border. By 1814, the county's jurisdiction had decreased to its present size and Bellevile was the center of population. As Cahokia was constantly threatened by floods, the county seat was moved to Belleville in 1811. The former courthouse became a town hall, storehouse, saloon, and again a home.