NY 104A spans three counties, serves two villages (Red Creek and Fair Haven) and parallels Lake Ontario for much of its length.

[3] North of Red Creek, NY 104A continues on a northerly alignment for another 3 miles (5 km), roughly paralleling the nearby Wayne–Cayuga county line.

East of the village, it continues on a generally linear routing across farmland to a junction with the northern terminus of NY 38.

The north–south alignment gradually brings NY 104A closer to Lake Ontario, which the route has loosely paralleled from Fair Haven eastward.

Here, the route passes through more farmlands and forests in the town of Oswego as it parallels the Lake Ontario shoreline 2 miles (3.2 km) to the northwest.

[3] For this reason, all of NY 104A is part of the Seaway Trail, a National Scenic Byway that extends across most of Upstate New York.

[8][9] US 104 was assigned in April 1935, extending from Niagara Falls to the town of Mexico by way of several previously numbered highways.