State Highway 171 is a southeast to northwest state route that starts by branching off U.S. Highway 84 in Mexia (southern end) and ends at an intersection with U.S. Highway 180 in Weatherford (northern end). The route was originally designated by 1933 along the portion between Hillsboro and Coolidge. By 1939, the route had been extended to the northwest significantly, absorbing portions of SH 2A and all of SH 122.