Illinois 18 crosses the Illinois River at Henry, and is the first river crossing south of Interstate 180. The terrain is generally flat outside of the Illinois River Valley area. In Streator, Illinois 18 westbound becomes Main Street. Eastbound, it becomes Bridge Street.
From 1918 through 1927, SBI Route 18 ran from Chicago to Princeton using modern U.S. Route 34. In 1939, it was applied to what was then SBI Route 17. The next year, the route was completed and Illinois 17 moved to a new highway south of Streator.