Its southern terminus is at Battery Place near the northern end of the Brooklyn–Battery Tunnel in New York City, where it intersects with both the unsigned Interstate 478 (I-478) and FDR Drive.

NY 9A was extended south into New York City in 1934 and north to Ossining in the late 1930s.

NY 9A was extended northward to Peekskill in 1967 following the completion of the Croton Expressway and southward to the Brooklyn–Battery Tunnel in the mid-1990s.

NY 9A begins in Lower Manhattan at Battery Place near the north end of the Brooklyn–Battery Tunnel (unsigned I-478) and heads north on the West Side Highway and Henry Hudson Parkway, crossing US 9 for the first time at the east end of the George Washington Bridge.

[4] NY 9A separates from US 9 along Ashburton Avenue and heads north as Saw Mill River Road.

It parallels the Saw Mill River Parkway through Ardsley and Elmsford, to the west side of Hawthorne.

The highway heads north along Riverside Avenue and eventually joins old Albany Post Road.

After crossing US 9 once more in Cortlandt, NY 9A ends at the Welcher Avenue interchange in southern Peekskill.

[8] In 1932, the New York Automobile Club drafted a plan establishing alignments for several routes through the city.

In this plan, NY 9A went south through the Bronx and into Manhattan on Broadway while US 9 used Riverdale Avenue north of 230th Street.

[10] In the final plan implemented in mid-December 1934, no route was assigned to the Harlem River Drive–Amsterdam Avenue corridor.

NY 9A ran south along the west side of Manhattan on Riverside Drive and the West Side Elevated Highway (detouring around an unfinished section via 57th Street, Eleventh Avenue and 48th Street) to end at the entrance and exit plazas of the Holland Tunnel.

[31] NY 9A was shifted onto 12th Avenue, one of the surface streets that the Elevated Highway had run atop of, but was otherwise unaffected as the route's south end was initially kept at the Lincoln Tunnel.

[33][34] Construction began in early 1996 on a project to convert the section of NY 9A south of 59th Street into the West Side Highway, a six-lane urban boulevard with a parkway-style median and decorative lightposts.

[35] Completion of the project was originally set for October 2001; however, it was delayed for years due to damage caused by the September 11 attacks.