Illinois State Route 31 runs north-south in Eastern Mc Henry County. This runs from the Kane/Mc Henry County Line, at Algonquin, to U.S. Rt. 12, at Richmond. IL St Rt 31 passes through the following towns in Mc Henry County. Algonquin Lake in the Hills Cary Crystal Lake Prarie Grove Mc Henry Johnsburg Ringwood Richmond
NB IL St Rt 31 in Algonquoin. A Mc Henry County mile marker. NB IL St Rt 31 at U.S. Rt. 14. A junction marker on SB U.S. Rt. 12. The northern end of IL St Rt 31.