In the late 1920s, what is now NY 17A from Greenwood Lake to Goshen was designated as part of NY 55, a connector between NY 17 in Goshen and the New Jersey state line. In the 1930 renumbering of state highways in New York, NY 55 was incorporated into two longer routes: NY 210 from New Jersey to Greenwood Lake and NY 17A from Greenwood Lake to Goshen. The modern routing of NY 17A between Greenfield Lake and Southfields was originally only part of NY 210, which continued past NY 17 and into Rockland County. NY 17A was extended east to Southfields by 1933, completing the alternate loop of NY 17 and creating an overlap with NY 210.