It begins in Hopkinsville, Kentucky and ends at the Ohio River, the Kentucky-Illinois state line in northern Crittenden County.

It goes on a northwesterly path, and its junction with Kentucky Route 398 is KY 91's access point to Pennyrile Forest State Resort Park.

[2] KY 91 enters Caldwell County, and then it would meet Kentucky Routes 139 and 293, along with US 62 in downtown Princeton.

KY 70 departs from US 641/KY 91 at a point north of the Crittenden-Caldwell County line, while KY 91 remains with US 641 until the U.S. Route's northern terminus at the U.S. Route 60 junction at Kentucky.

For a few blocks, KY 91 runs concurrently with US 60, and then KY 91 goes further north to reach its northern terminus at the Cave-In-Rock Ferry on the Ohio River, which also marks the Kentucky-Illinois state line.