Its northern terminus is at the Canada–United States border in Ogdensburg, where it crosses the Ogdensburg–Prescott International Bridge and connects to Highway 16 in Ontario.

While most of NY 812 passes through rural areas of the North Country, the route also serves several villages and small communities.

The GPS services company Geotab has named it the quietest highway in New York due to its low traffic counts.

Just before Bostwick ends at East State Street, NY 812 turns off and begins its progression northward through a largely rural area of the North Country.

As it exits the village of Lowville (but remains in the town of the same name), it begins to parallel the western bank of the Black River.

The two entities remain close for roughly 1 mile (1.6 km) before NY 812 crosses over the river and enters New Bremen.

[4] In the hamlet of New Bremen, NY 812 intersects County Route 33 (CR 33), a riverside roadway bypassing both Croghan and Beaver Falls along the east bank of the Black River, southwest of Duflo Airport.

NY 812 continues onward, crossing the Black Creek as it enters Croghan, a village situated on the New Bremen–Croghan town line.

The routes quickly exit the village and separate shortly thereafter south of the hamlet of De Kalb, with the Oswegatchie leaving the path of US 11 and following NY 812.

The first change to the route's southern end came in the early 1940s when it was moved northeastward to the eastern terminus of its former overlap with NY 58 in Edwards.

[12][13] In 1975, officials from New York State Department of Transportation (NYSDOT) Region 7 proposed eliminating NY 87 in favor of NY 812, a new route that would extend from Lowville to Ogdensburg via Croghan, Harrisville, Gouverneur, and De Kalb.