NY 840 begins at an intersection with Halsey Road (CR 52) and Judd Road in the town of Whitestown. North of this junction, NY 840 continues as Judd Road, designated as CR 840. NY 840 initially heads south as a four-lane undivided highway through a rural portion of Oneida County to a junction with Clark Mills Road (CR 19), where it turns southeast to parallel a set of high-voltage power lines through the town. Upon crossing into the town of New Hartford, NY 840 becomes a limited-access highway and turns again to follow a more easterly routing.