The southern terminus of the route is located at an intersection with NY 13 in the Cortland County town of Truxton.

NY 91 originally extended as far south as Cincinnatus and as far north as Bridgeport when it was assigned as part of the 1930 renumbering of state highways in New York.

NY 91 proceeds north across and along Labrador Creek, crossing a junction with the terminus of County Route 151 (Prospect Street).

[3] Continuing through the town of Fabius, NY 91 remains the two-lane rural roadway, turning westward at a junction with Cemetery Road (CR 170).

This western stint is short, with NY 91 turning north again at a junction with the eastern terminus of Chase Road (CR 237A).

A short distance later, both routes reach respective junctions with NY 173 (East Seneca Turnpike) in the town of De Witt.

[3] When NY 91 was assigned as part of the 1930 renumbering of state highways in New York,[2] it was substantially longer than it is today.