Wyoming Highway 91 begins its western end at Converse County Route 11 southwest of La Prele and near the Medicine Bow National Forest boundary.

Highway 91 proceeds northeast as Cold Springs Road and through the community of La Prele.

Almost at the Douglas city limits, Interstate 25 comes into view to the north and is paralleled by Highway 91 to its end.

With the introduction of U.S. routes in 1926; The routing between Cokeville and Star Valley was renamed Wyoming Highway 89, while the section south of Sage remained Wyoming Highway 65.

Highway 89 was numbered in hopes that U.S. Route 89 (which had a northern terminus in central Utah at that time) would be extended north into Wyoming.