Illinois 171 runs north-south largely near and to the west of Chicago, in the western and southwestern suburbs. It begins in Joliet, loosely following the east side of the Des Plaines River, northeast to Summit. At Summit, the route turns from a southwest-northeast alignment into a straighter north-south route through Cook County's inner suburbia. Finally, Illinois 171 ends just east of O'Hare, in the northwestern reaches of the Chicago city limits.
From south to north, Illinois 171 starts in Joliet at US Route 6: