The southern terminus of the route is at an intersection with NY 242 in the Ellicottville community of Ashford Junction in northern Cattaraugus County.

The northern part of NY 240 in Erie County, named Harlem Road, is a major north–south route through the suburbs east of the city of Buffalo.

NY 240 proceeds northward alongside Beaver Meadows Creek co-designated as County Route 32 (CR 32; West Valley Road).

NY 240 and CR 32 continue northeast as the dense woods retreat for residences before bending northwest away from Beaver Meadows Creek.

At that junction, the routes bend northwest again, passing a stretch of residences before intersecting with the western terminus of CR 16 (Roszyk Hill Road).

[3] At Twichell Road, NY 240 and CR 32 make a large bend to the west, crossing through Ashford before turning northeast and northwest.

NY 240 and CR 32 continue north through Ashford, intersecting with Beech Tree Road, where they cross Cattaraugus Creek just west of the Bigelow Bridge.

[3] Upon crossing into Erie County and the town of Concord, NY 240 remains county-maintained, now co-designated as unsigned CR 198.

Now known as South Vaughn Street, the route proceeds northwest, intersecting with the eastern terminus of CR 62 (Cattaraugus Road).

The route bends northward, entering the village of Springville, where it intersects with NY 39 (East Main Street / Creek Road).

[3] NY 240 and CR 198 continue northward as a two-lane rural roadway in Concord, remaining with the moniker of Vaughn Street.

NY 240 and CR 30 proceed northward through Concord, soon bending northwest through dense fields before returning to railroad tracks nearby.

The routes bend northward through the hamlet of Footes, paralleling the creek and railroad track before entering the town of Colden.

In Colden, NY 240 and CR 30 become a two-lane residential street surrounded by dense woods in the nearby hills.

The woods changeover to wide fields as it winds northwest, passing west of Walter Kummer Town Park before entering the hamlet of Colden.

After Philson Street, the route makes a short bend to the west, intersecting with NY 277 (Chestnut Ridge Road).

For a short distance, NY 240 serves as a two-lane commercial boulevard, switching over to a residential street at Dover Drive.

Now in the town of West Seneca, NY 240 retains its Orchard Park Road moniker, passing several condominium complexes on both sides of the highway.

NY 240 proceeds north through West Seneca, passing several residences and businesses before crossing over Cazenovia Creek into a large residential district.

Just northeast of exit 53, NY 240 crosses under the Thruway mainline, intersecting with the western terminus of CR 315 (Dingens Street).

Passing just east of the village of Sloan, NY 240 proceeds northward along Harlem Road over a railroad yard owned by CSX Transportation.

A short distance north, the route passes east of Ridge Lawn Cemetery, crossing under NY 33 (the Kensington Expressway).

Continuing north through Cheektowaga, NY 240 becomes a two-lane residential street, intersecting with CR 141 (Cleveland Drive) at a rotary.

The route remains residential for a distance to the intersection with Campus Lane, where it enters exit 6 of I-290 (the Youngmann Expressway).