It extends for 34.83 miles (56.05 km) from an intersection with New York State Route 51 (NY 51) in the town of Butternuts to the Oneida County line at Plainfield, where it becomes CR 2.

The portion of CR 18 between Unadilla Forks and U.S. Route 20 (US 20) in Plainfield—a distance of 2.52 miles (4.06 km)—was originally maintained by the state of New York.

CR 18 begins at an intersection with NY 51 in the town of Butternuts, located east of the hamlet of Mount Upton on the eastern bank of the Unadilla River.

[citation needed] North of Lathams Corners, CR 18 serves as a divider between a series of cultivated fields on the east bank of the river and a dense forest enveloping the eastern edge of the valley.

Past the intersection, CR 18 passes through the eastern extents of the hamlet before continuing northeastward through open fields.

While NY 8 directly enters the village, which is situated on the west bank of the river, CR 18 bypasses it to the east.

The route crosses over Wharton Creek and intersects NY 80 in an area known as Hoboken before leaving the vicinity of New Berlin.

Roughly 2 miles (3.2 km) north of Leonardsville, CR 18 enters the small hamlet of Unadilla Forks.