It serves as the main east–west route for most of the North Shore of Long Island, running for 73 miles (117 km) from Interstate 495 (I-495) at the Queens–Midtown Tunnel in the New York City borough of Queens to NY 25 in Calverton, Suffolk County.

The highway serves the Long Island City Courthouse on its way to a junction with Queens Boulevard (NY 25) at the foot of the Queensboro Bridge.

As Northern Boulevard, NY 25A begins to follow a more easterly path, loosely paralleling the Sunnyside Yard to the south.

NY 25A becomes a part of a large street grid and running along a linear alignment through Jackson Heights, Corona, and East Elmhurst.

It eventually reaches Flushing Meadows-Corona Park, where the highway intersects with the Grand Central Parkway just northwest of Citi Field.

[4][5] NY 25A, still bearing the Northern Boulevard name, crosses into Nassau County at the Great Neck hamlet known as University Gardens.

The route continues eastward, intersecting with NY 101 and Nassau County Route 101 (Port Washington Boulevard and Searingtown Road, respectively) just south of Port Washington, in the villages of Flower Hill (on its north side) and Roslyn Estates (on its south side), ahead of a split in Northern Boulevard.

It briefly reverts to four lanes at Cold Spring Road, and the highway enters Suffolk County less than a mile (1.6 km) later.

At an intersection with County Route 35 (CR 35) on the east side of Huntington, NY 25A narrows to two lanes once more and heads toward Centerport, where it becomes Fort Salonga Road.

This name remains in place through Centerport, Northport and Fort Salonga to the vicinity of Sunken Meadow State Park.

NY 25A continues on an eastward track to San Remo (just east of Kings Park), where it makes a sharp southward turn at an intersection with St. Johnland Road in the center of the community.

The Jericho Turnpike name ends a short distance into the community at a junction with Edgewood Avenue, giving way to Main Street.

On the eastern edge of the Stony Brook campus, the route intersects with the north end of CR 97 (Nicolls Road).

They reconnect east of Rocky Point to continue toward Shoreham and an intersection with CR 46 (William Floyd Parkway) near Brookhaven State Park.

The route in Queens and Nassau County was originally an east–west Indian trail between the current town of North Hempstead and Flushing.

The old route is now known as North Country Road from Port Jefferson Station to Sound Beach (formerly CR 20) and from Shoreham to Wading River.

[13] In the 1960s, NY 25A was extended southwestward along Jackson Avenue to meet the Long Island Expressway (I-495) just east of where it enters the Queens–Midtown Tunnel.

[20][21] The construction of the Roslyn Viaduct in the 1940s, for example, enabled traffic to flow more freely over Hempstead Harbor and keeping the historic community to the south preserved intact.

Only the Roslyn Viaduct and the Rocky Point Bypass were constructed; other attempts to realign the highway were cancelled as a result of public opposition.

The projects were largely opposed out of fear that they would lower property values and bring more traffic jams and rampant development.