It runs from Interstate 70 in Altamont to Illinois Route 121 in Dalton City.

[1] Illinois 128 is a two-lane, rural surface road for its entire length.

Immediately north of Interstate 70, it overlaps U.S. Route 40 for 3 miles (5 km).

Illinois 128 may or may not run through Moultrie County, depending on its exact routing, as the route appears to run along portions of the Shelby/Moultrie and Macon/Moultrie county lines.

In March 1937, it replaced IL 169 north of Shelbyville to Dalton City.