NY 111 begins at an intersection with
NY 27A (Main Street) in the town of
Islip. The route proceeds north through the
hamlet of Islip along the two-lane Islip Avenue, passing through a short residential stretch ahead of an intersection with
CR 50 (Union Boulevard). NY 111 soon crosses the nearby
Montauk Branch of the
Long Island Rail Road just east of the
Islip station, from where it continues north past various businesses and residences. As the road heads northward, it intersects with Bittermint Street, a connector to
NY 27 (
Sunrise Highway) eastbound. After crossing over NY 27, the route intersects with a ramp serving the westbound lanes and Islip Boulevard, which connects NY 27 westbound to NY 111. Past Sunrise Highway, the route serves many blocks of residences prior to entering a
cloverleaf interchange with the
Heckscher State Parkway (exit 43). After this junction, NY 111 continues north as a two-lane residential street to the
hamlet of
Central Islip. Here, the residences give way to various businesses as NY 111 and Islip Avenue briefly bend to the northwest for several blocks. Another north-northwesterly stretch follows, bringing the highway over another Long Island Rail Road line, this time the
Ronkonkoma Branch. Past the railroad overpass, NY 111 intersects with
CR 100 (Suffolk Avenue), at which point NY 111 leaves Islip Avenue and changes names to Joshua's Path. One block north of CR 100, Caleb's Path forks off to the northwest while NY 111 splits to the northeast along Joshua's Path, serving a long stretch of homes in the northern part of Central Islip. Several blocks to the north, NY 111 intersects with
CR 67 (
Long Island Motor Parkway) at an at-grade intersection on the northern edge of the community. Now out of Central Islip, the route traverses industrialized areas surrounding a large
Y intersection with
CR 17 (Wheeler Road) in
Hauppauge. NY 111 takes on the Wheeler Road name, and the route immediately enters an interchange (exit 56) with the
Long Island Expressway (Interstate 495). From here, the route bends northeast along Wheeler Road, paralleling the
Nissequogue River as it serves the northern part of the town of Islip. The highway passes the
American Real Estate School and a campus of
Empire State College before intersecting
NY 454 (Veterans Highway). Not far from NY 454, NY 111 has a short concurrency with
CR 76 (Townline Road) along the Islip–
Smithtown town line and then the names changes to Hauppauge Road before fully crossing into Smithtown. After forking from CR 76, NY 111 continues northeast through the hamlet of Hauppauge, crossing
NY 347 (Smithtown Bypass) at-grade as it runs across another residential stretch. The route soon enters
Village of the Branch, where it bends northward into a large commercial area at the center of the community. Here, NY 111 intersects with East Main Street, which enters the junction from the west as
NY 25 and
NY 25A. At this point, NY 25 continues eastward as Middle Country Road, NY 111 terminates, and NY 25A turns northward on NY 111's right-of-way, changing names to North Country Road. ==History==