It runs from South Meyer Avenue & C Road in Valmeyer to just west of New Athens.

This is a distance of 22.95 miles (36.93 km).

It is the main east–west state road through Waterloo.

Illinois 156 overlaps Illinois Route 159 from Hecker north for a short distance, when 156 leaves 159 and turns back toward its eastern terminus.

SBI Route 156 is the same as Illinois 156 is now.