NY 18 was assigned in 1924 and originally extended from the Pennsylvania state line near Salamanca to downtown Buffalo via Dayton and Hamburg.

It was extended northeast to Rochester via Niagara Falls as part of the 1930 renumbering of state highways in New York and east to NY 250 in the town of Webster by the following year.

After leaving Niagara County, NY 18 shifts farther south, gradually moving away from the shore of Lake Ontario.

The parkway then becomes the lakeside road, and NY 18 veers south to follow a more inland routing.

After 0.5 miles (0.8 km), the highway curves to the north and meets NY 104 at an unconventional grade-separated interchange that has a pair of two-way ramps connecting the two state routes.

Southwest of the Four Mile Creek State Park, NY 18 turns a full 90 degrees to the east and begins to parallel the southern shore of Lake Ontario.

The route proceeds northeast to Four Mile Creek State Park, where it intersects the northern end of the Niagara Scenic Parkway.

While NY 63 ends here,[3] Lyndonville Road continues north to the lake as County Route 63-1 (CR 63-1).

The junction is the northernmost exit on NY 390 prior to its merging with the Lake Ontario State Parkway to the north.

[3] A small distance east of NY 390, NY 18 passes Greece Arcadia High School and intersects Mount Read Boulevard in the hamlet of Mount Read before intersecting Dewey Avenue a half-mile to the east.

NY 18 turns south onto Dewey Avenue;[3] however, state maintenance continues to follow Latta Road east to where it crosses into the Rochester city limits at Charlotte.

[7] NY 18, meanwhile, becomes maintained by Monroe County as part of CR 132, an unsigned designation that follows Dewey Avenue north to its end at the Lake Ontario shoreline.

[2][11] NY 18 was extended northeastward to Rochester as part of the 1930 renumbering of state highways in New York, effectively doubling the route's length.

[12][14] The majority of NY 18's routing south of the village of Lewiston was incorporated into the U.S. Highway System in the early 1930s after US 62 and US 219 were extended into New York and US 104 was assigned.

NY 18 now began concurrent to US 219 at the state line and overlapped the route northward to the city of Salamanca.

[22][23] In the Buffalo area, NY 18 was realigned twice in the 1930s: first by 1935 to bypass downtown to the east on Bailey Avenue between Abbott Road and Main Street[19] and again in the late 1930s to use Bailey Avenue and Eggert Road between Main Street and Niagara Falls Boulevard.

[22][23] NY 18 was rerouted between Lewiston and Youngstown on January 1, 1949, to follow a more inland highway through western Niagara County.

The route turned north, following the highway to its end at Culver Road, where it rejoined its pre-expressway alignment.

[33] The overlap proved to be temporary as NY 18 was truncated westward c. 1973 to its current eastern terminus in Kodak Park.

[36] East Ridge Road, meanwhile, was now devoid of any designations as US 104 had been shifted onto the Keeler Street Expressway several years before.

[33][37] As a result, ownership and maintenance of the Irondequoit section of East Ridge Road was transferred to Monroe County, which designated it as the unsigned CR 241.

[38] The alignment of NY 18 through Olcott Beach dated back to the early 1800s, starting with a foot bridge over Eighteen Mile Creek in 1811, built out of wood.

This was replaced by a white oak frame bridge built in 1825 for $500 (1825 USD) for the use of transporting wagons across the creek.

General James Weisner contracted to build the bridge, which was 77 feet (23 m) wide, along with 20 20-foot (6.1 m) long spans.

This span was too low to the water line, causing a bottleneck of boats and ships entering Eighteen Mile Creek.

This new bridge would involve building an approach at West Creek Road near the local water tower.

The alignment would then cross Franklin Street and through nearby Krull Park where it would meet then-current NY 18.

[47] The process was so rapidly advancing that despite the late delivery of steel in October 1969 for the new structure, it failed to stop the construction.

[50] The Army Corps of Engineers expanded navigation of Eighteen Mile Creek to Burt in February 1972 thanks to bridge construction.