Prior to the establishment of the
U.S. Highway System,
US 9 was designated as
NY 6. An alternate route from
Yonkers to
Tarrytown was assigned the NY 6A designation by 1926. This ran along the present alignment of NY 9A from Yonkers to north of
Elmsford, where it turned west on Old Saw Mill River Road, Neperan Road, County House Road and Bedford Road to end at NY 6 in Tarrytown. NY 6 was redesignated as US 9 when U.S. Highways were first posted in New York in 1927; however, NY 6A was not renumbered at this time. It was finally renumbered to NY 9A as part of the
1930 renumbering of state highways in New York. In
Westchester County, Saw Mill River Road originally followed the
Saw Mill River Parkway corridor from
Eastview to
Hawthorne. This section of Saw Mill River Road gained a number , becoming part of NY 142, a route that began at
NY 100 on the
Greenburgh–
Mount Pleasant town line and followed Grasslands Road, NY 9A, and Saw Mill River Road north to Hawthorne, where it rejoined NY 100. The route went unchanged until it was removed . Its former routing was split into two routes—an extended
NY 141 north of NY 9A and the new
NY 100C along Grasslands Road—by 1940. As the
Henry Hudson Parkway replaced
Riverside Drive in the mid-1930s, NY 9A was moved onto it, eventually using the new parkway to where it crossed US 9 (Broadway) in the Bronx. Here, NY 9A exited the parkway and ran
concurrent with US 9 to the split in
Yonkers. The
Brooklyn–Battery Tunnel connecting Manhattan's
Battery Park to
Brooklyn was completed in 1950. NY 9A was extended northward from Tarrytown to Archville, a small
hamlet midway between Tarrytown and Ossining, by way of Sleepy Hollow Road, in 1938. Farther north, a new
limited-access parkway was built around
Ossining from Saw Mill River Road (NY 100) in
Briarcliff Manor to US 9 north of Ossining. The parkway, known as the Briarcliff–Peekskill Parkway, was completed and initially designated as NY 404. On January 1, 1949, NY 9A was altered to continue north along a slightly realigned Saw Mill River Road from NY 100C near
Elmsford to the south end of the Briarcliff–Peekskill Parkway in Briarcliff Manor. At this point, NY 9A left Saw Mill River Road and followed the parkway to its end at US 9, supplanting NY 404. The realignment created a overlap between NY 9A and NY 100 from Hawthorne to Briarcliff Manor and resulted in the truncation of NY 141 back to its previous terminus in Hawthorne. Construction on the
Croton Expressway, the only piece of the failed
Hudson River Expressway project that was ever built, began in the mid-1960s.
New York City initially did not mark numbered routes within its limits. 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. As a result, the two routes would have had a short
concurrency across
Spuyten Duyvil Creek. NY 9A would have split to the south on
Tenth Avenue at
218th Street in order to join the
Harlem River Drive via
Nagle Avenue and
Dyckman Street. From there it would head west on
155th Street to
Amsterdam Avenue, where it would head south to
79th Street, heading west there to rejoin US 9 at
Riverside Drive. US 9 would have continued south through
lower Manhattan to
Staten Island via the
Staten Island Ferry; however, it is unclear whether NY 9A would have continued south with US 9 to lower Manhattan. The New York Automobile Club released another plan in 1933. This plan made no changes to NY 9A; however, US 9 was changed to use Broadway all the way through the Bronx and to travel to
New Jersey by way of the
Holland Tunnel. In the final plan implemented in mid-December 1934, no route was assigned to the Harlem River Drive–Amsterdam Avenue corridor. Instead, NY 9A used what had been planned as US 9, splitting at
Broadway and
Dyckman Street. 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. US 9 was shifted northward to enter New Jersey via the
George Washington Bridge. which had ended in Brooklyn prior to the construction of the tunnel. On January 1, 1970, NY 27A was truncated on its western end to eastern
Nassau County while
NY 27 was extended northward over NY 27A's former routing through the Battery Tunnel and the West Side Elevated Highway. The extension of I-478 into Manhattan was eliminated following the collapse of part of the Elevated Highway in 1973, an event which led to the demolition of the highway south of
59th Street. Demolition was completed in 1989. 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. However, by 1973, NY 27 had been cut back to its interchange with the
Gowanus Expressway in Brooklyn while unsigned
I-478 was assigned to both the Battery Tunnel and all of the West Side Elevated Highway south of the
Lincoln Tunnel. NY 9A was cut back to the Lincoln Tunnel as a result. 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. The first of the project's seven segments—between Clarkson and Horatio streets in the
Greenwich Village neighborhood—was completed in 1998. As part of this, NY 9A was extended south to the Battery Tunnel by way of 12th Avenue and two other streets the Elevated Highway had previously run atop of,
West Street and
11th Avenue. 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. It was finished by 2014. ==Major intersections==