It extends south from Wisconsin Highway 31 and Lake CR A1/19 (128th Street) at the Illinois/Wisconsin state line by Pleasant Prairie, south to Illinois Route 176 in Lake Bluff, a distance of 15.15 miles (24.38 km).

[1] Illinois 131 is called Green Bay Road for its entire length, but Green Bay Road becomes a residential street and extends south through various North Shore communities to Evanston, a distance of approximately 20 miles (32 km) south of Illinois 176.

Green Bay Road actually continues south to Old Elm Road (Lake CR 52) while there is a second segment of Green Bay Road that starts at a dead end in Highwood and ends at Ridge Avenue in Evanston.

The Green Bay Road name also stretches continuously through Racine County and Kenosha County, WI, with route signage changing to Wisconsin Highway 31, before merging with Wisconsin Highway 32 north of Racine.

SBI Route 131 ran from Mattoon to Greenup in southeastern Illinois.