Park Avenue is a boulevard in New York City that carries north and southbound traffic in the boroughs of Manhattan and the Bronx. For most of the road's length in Manhattan, it runs parallel to Madison Avenue to the west and Lexington Avenue to the east. Park Avenue's entire length was formerly called Fourth Avenue; the title still applies to the section between Cooper Square and 14th Street. The avenue is called Union Square East between 14th and 17th streets, and Park Avenue South between 17th and 32nd streets.

Because of its designation as the widest avenue on Manhattan's East Side, Park Avenue originally carried the tracks of the New York and Harlem Railroad built in the 1830s, just a few years after the adoption of the Manhattan street grid. The railroad's right-of-way at ground level forced foot and carriage traffic onto either side of the tracks. Later on, the railroad was run through an open cut tunnel under Murray Hill, which was then covered with grates and grass between 34th and 40th Street in the early 1850s. A section of this "park" was later renamed Park Avenue in 1860. Park Avenue's original southern terminus was at 34th Street, and the newly renamed Park Avenue was given its own house-numbering system separate from that of Fourth Avenue. The address 1 Park Avenue was assigned to a house at 101 East 34th Street, at the northeast corner of Park Avenue and 34th Street.

The Harlem Railroad was later incorporated into the New York Central Railroad, and a terminal for the New York Central at 42nd Street, the Grand Central Depot, opened in 1871. But the tracks laid to the new terminal proved problematic. There were originally no grade-separated crossings of the railroads between 42nd and 59th Streets. As such, they required railroad crossings along Fourth Avenue, which resulted in frequent accidents; seven people died within 12 days of the Hudson River Railroad's move to Grand Central.

In 1872, shortly after the opening of Grand Central Depot, New York Central owner Cornelius Vanderbilt proposed the Fourth Avenue Improvement Project. The tracks between 48th and 56th Streets were to be moved into a shallow open cut, while the segment between 56th and 97th Streets, which was in a rock cut, would be covered over. After the improvements were completed in 1874, the railroads, approaching Grand Central Depot from the north, descended into the Park Avenue Tunnel at 96th Street and continued underground into the new depot. As part of the project, Fourth Avenue was transformed into a boulevard with a median strip that covered the railroad's ventilation grates. Eight footbridges crossed the tracks between 45th and 56th Streets, and there were also vehicular overpasses at 45th and 48th Streets. The boulevard north of Grand Central was renamed Park Avenue in 1888.

A fatal collision between two trains occurred under Park Avenue in 1902, in part because the smoke coming from the steam trains obscured the signals. The New York state legislature subsequently passed a law to ban all steam trains in Manhattan. By December 1902, as part of an agreement with the city, New York Central agreed to put the approach to Grand Central Station from 46th to 59th Streets in an open cut under Park Avenue, and to upgrade the tracks to accommodate electric trains. Overpasses would be built across the open cut at most of the cross-streets. The new electric-train terminal, Grand Central Terminal, was opened in 1913.

After the electric trains were buried underground, the area around Park Avenue in the vicinity of Grand Central was developed into several blocks worth of prime real estate called Terminal City. Stretching from 42nd to 51st Streets between Madison and Lexington Avenues, it came to include the Chrysler Building and other prestigious office buildings; luxury apartment houses along Park Avenue; and an array of high-end hotels that included the Marguery, Park Lane, and Waldorf Astoria. In 1929, New York Central built its headquarters in a 34-story building (now called the Helmsley Building), straddling Park Avenue north of the terminal.

The Park Avenue Viaduct reroutes Park Avenue around Grand Central Terminal between 40th and 46th Streets, allowing Park Avenue traffic to traverse around the building and over 42nd Street without encumbering nearby streets. The western (now southbound) leg of the viaduct was completed in 1919, but congestion developed soon after the viaduct's opening, so an eastern leg for northbound traffic was added in 1928.

The developer Henry Mandel acquired the lots on the eastern side of Fourth Avenue between 32nd and 33rd Street in 1923 under the name "One Park Avenue Corporation". To ensure his corporate name was accurate, Mandel asked the New York City Board of Aldermen to move Park Avenue's southern terminus to 32nd Street. The change went into effect on December 1, 1924, and address numbers along Park Avenue were changed accordingly. The previous house numbered 1 Park Avenue was occupied by Martha Bacon, widow of diplomat Robert Bacon, who led the opposition to the renumbering. The Board of Aldermen summarily overturned the name change, but Mayor John Hylan vetoed the move in April 1925. This prompted Bacon to appeal the decision to the New York Supreme Court, Appellate Division, which overturned Hylan's veto in November 1927, on the basis that the extension of Park Avenue to 32nd Street had been made for the benefit of a developer. Mandel's development at 32nd Street was thus known as 461–477 Fourth Avenue, and the developers of that building sued to reverse the appellate ruling. The New York Court of Appeals, the state's highest court, reversed the appellate ruling in February 1928. Bacon contemplated bringing up the matter with the United States Supreme Court, but she ultimately relented, changing her address to "Park Avenue at 34th" by 1930.