NY 420 was originally part of Route 32, an unsigned legislative route created by the New York State Legislature in 1908. The Winthrop–Massena portion of Route 32 became part of the signed NY 56 in the mid-1920s; however, NY 56 was realigned in 1927 to follow a new alignment well to the south of Winthrop. The former routing of NY 56 between Winthrop and Massena was redesignated as NY 420 as part of the 1930 renumbering of state highways in New York. NY 420 was extended southward to its current terminus south of Winthrop in 1980.