The western terminus of the route is at an intersection with NY 14 at New York State Thruway (Interstate 90 or I-90) exit 42 in the town of Phelps.

The origins of NY 318 date back to the 19th century when the highway was part of a road connecting the village of Phelps to a bridge over the Seneca River near Cayuga Lake.

West of the intersection, the right-of-way for NY 318 leads to the toll barrier for New York State Thruway (I-90) exit 42.

The route heads eastward, crossing the Norfolk Southern Railway Corning Secondary line before passing into Seneca County[4] a mere 0.70 miles (1.13 km) from NY 14.

Past the outlet mall, the route continues through largely rural surroundings to the community of Stone Church Corner, where it briefly curves to the northeast before veering back to the east as it enters the town of Tyre.

East of Magee, NY 318 turns to the southeast and begins to separate from the Thruway as it passes through vast, open fields.

At the community of Nichols Corners, located at the intersection of NY 318 and Gravel Road (CR 101), the route curves back to the east and begins to straddle the Tyre–Seneca Falls town line.

[4] On April 16, 1825, the New York State Legislature passed an act permitting the construction of a "free bridge" over the Seneca River.

[9] The entirety of old Route 6-a between West Junius and Halsey Corners was assigned a designation as part of the 1930 renumbering of state highways in New York.