East of Oakdale, NY 27 originally followed Montauk Highway to Montauk Point. NY 27A was extended eastward along Montauk Highway to Patchogue in the early 1950s after NY 27 was realigned to follow a new highway between Oakdale and Patchogue. A limited-access highway extension of NY 27 to Shirley opened to traffic in the late 1950s, at which time NY 27A was extended eastward along NY 27's former routing to meet NY 27 in Shirley. Construction on a new alignment for NY 27 between NY 24 in Hampton Bays and Montauk Highway near Water Mill began by 1961 and was completed . The former routing of NY 27 between the two locations became part of NY 27A, even though it did not connect to any other part of the route.
