The road on which Thompson Mill Bridge stands was once an important route between Springfield and Effingham. The bridge was completed in the autumn of 1868 at a cost of $2,500 and was named for the owner of the first mill nearby. It is the narrowest covered bridge in Illinois, measuring only 10 feet 7 inches wide and 11 feet 4 inches high. Its siding stops before reaching the top cord so light can enter beneath the roof. The bridge uses the Howe Truss System, made up of panels in which two members cross one diagonal, and its truss is 105 feet long. The roof protected the truss from the weather.