[4] At the intersection with CR 1, NY 27A and Merrick Road cross into Suffolk County and the village of Amityville in the town of Babylon.

Once again a four-lane boulevard, NY 27A crosses into the village of Lindenhurst, now boasting the West Montauk Highway moniker.

[4] NY 27A through Lindenhurst remains commercial, passing several ocean-side marinas on the eastbound side of the highway.

At the intersection with the southern terminus of CR 3 (South Wellwood Avenue), the name flips to East Montauk Highway as NY 27A continues northeast through Lindenhurst.

At Park Avenue, the route passes south of a long strip mall along the north side of the roadway, crossing into the village of Babylon.

Now in Babylon, the route switches back to West Montauk Highway once again, crossing an intersection with CR 96 (Great East Neck Road) and through the Bergen Point Golf Course.

Bending a little to the east, NY 27A becomes a four-lane boulevard across the southern end of Argyle County Park.

[4] Now named Montauk Highway, NY 27A bends southeast past an intersection with the southern end of CR 82 (Higbie Lane).

The route crosses east through Islip, passing south of Lake Capri, and bending northeast at Everdell Avenue.

After passing Captree Elementary School, NY 27A enters exit RM2 of the Robert Moses Causeway, crossing into West Bay Shore on the other side of the parkway.

Running northeast for a short distance through West Bay Shore, NY 27A winds past Southward Ho Country Club as a two-lane residential road.

Now in Great River, NY 27A bends northeast as Montauk Highway, junctioning with the eastern terminus of CR 50 (Union Boulevard).

After crossing into North Great River, the route winds through the park as a two-lane roadway, paralleling the Montauk Branch of the Long Island Rail Road.

The Suffolk County portion of its former alignment on Merrick Road and Montauk Highway was designated as NY 27A.

[7] The route remained on Merrick Road to the Suffolk County line, where it joined its original alignment.

[13] Ownership and maintenance of Montauk Highway between Great River and the town of Southampton was gradually transferred from the state of New York to Suffolk County during the 1960s and 1970s.

The project, dubbed the "Bay Shore Split", would reroute eastbound and westbound traffic on new alignments running to the north and south of Main Street (NY 27A) and convert Main Street into a pedestrian mall with parking garages at key locations.