The initial northern terminus of NY 420 was in the center of Massena at the junction of Orvis and Main Streets, where it ended at NY 37. In the mid-1950s, construction began on a new alignment for NY 37 around the southern edge of the village. The highway opened to traffic by 1958; however, NY 420 continued to extend past the bypass into Massena until the early 1960s, when it was cut back to its junction with NY 37's new routing. The former routing of NY 420 in Massena is still state-maintained as part of NY 970B, an unsigned reference route designation for all of Main Street between NY 37 and NY 37B.