It runs east from the town of Henry at Illinois Route 29 to the town of Blackstone at Illinois Route 17.

[1] 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.

[5][6] The next year, the route was completed and Illinois 17 moved to a new highway south of Streator.