NY 420 begins at an intersection with US 11 in the town of Stockholm. The route heads north as a two-lane highway through forested areas and across both branches of the St. Regis River to the hamlet of Winthrop, the first area of significant development along the highway. Here, the two branches of the St. Regis River converge and NY 420 meets NY 11C. NY 420 turns northeastward, overlapping NY 11C for two blocks before splitting from it in the center of Winthrop.