Numerous elevators make most of the complex, except for the Lexington Avenue Line station, compliant with the Americans with Disabilities Act of 1990 (ADA).

It called for a subway line from New York City Hall in lower Manhattan to the Upper West Side, where two branches would lead north into the Bronx.

[7] Two streetcar tracks on Union Square East were temporarily relocated to one side of the street while contractors excavated through solid rock on the site.

[8] During the line's construction, the contractors installed a temporary compressed-air plant in Union Square, prompting a lawsuit from the operator of a nearby hotel.

[9] The New York Court of Appeals ruled in 1902 that the contractors had to disassemble the compressed-air plant and to stop storing materials in Union Square Park.

[18] As part of an experiment to improve the subway line's ventilation, the Rapid Transit Commission installed large fans at the 14th Street station in July 1905.

[19][20] To address overcrowding, in 1909, the New York Public Service Commission proposed lengthening the platforms at stations along the original IRT subway.

[21]: 168  As part of a modification to the IRT's construction contracts made on January 18, 1910, the company was to lengthen station platforms to accommodate ten-car express and six-car local trains.

[5]: 203–219 [27] In May 1913, the Public Service Commission began receiving bids for Section 4 of the Broadway Line, between Houston Street and Union Square.

[31][30] Local civic group Broadway Association and various property owners objected to the fact that Dock Contractor was to receive the contract, citing the firm's lack of experience.

[50][51] To aid navigation, in 1920, the BRT installed illuminated signs on the southbound platforms of Union Square and two other Broadway Line stations.

[71][72] In May 1928, the New York City Board of Estimate awarded a $607,223 contract to improve transfers between the Canarsie and Broadway lines at Union Square,[73] which was expected to help relieve crowding at Canal Street.

[75] The project involved constructing a 30,000-square-foot (2,800 m2) mezzanine[75] and raising and re-landscaping the adjacent portion of Union Square Park to provide enough headroom.

[86][87] The BOT studied the feasibility of building an underpass from the station to the eastern side of Union Square East at 15th Street in January 1949,[88][89] and Loft Inc. opened a candy store in the BMT mezzanine that April.

[92] The New York City Transit Authority (NYCTA) lengthened both of the Lexington Avenue Line platforms by 150 feet (46 m) as part of a $1.3 million project that was completed at the beginning of 1958.

After the extensions were completed, the NYCTA began requiring that, during PM rush hours, alighting Lexington Avenue Line riders use only one of the staircases on the northbound platform to reduce congestion.

[95] Additionally, the NYCTA installed a closed-circuit television system on the Lexington Avenue Line platforms in 1965 as part of a pilot program to deter crime there.

[101] Local civic groups, the MTA, and the New York City Transit Authority began soliciting bids for the station's renovation in December 1978.

[121] On August 28, 1991, an accident just north of the IRT station killed five passengers and injured 215 others in one of the deadliest incidents in New York City Subway history.

As part of the contract, the consultant investigated whether the MTA could reconfigure the IRT passageway, reframe the exit structure on the Lexington Avenue platforms to accommodate the relocation and widening of stairs, build a new fan room, remove stairs on the Broadway Line platforms, reframe the existing structure, and rebuild a new staircase between the intermediate and IRT mezzanines.

[129]: C-57  Lee Harris Pomeroy prepared plans for the project, which was to cost $38.5 million and start in December 1994, with a new entrance pavilion and elevator on the southeast corner of Union Square Park.

[136] During the late 1990s, the MTA had received $1.6 million from The Related Companies, which had developed a building on the site of two subway entrances at Broadway and 14th Street.

[137] Subsequently, mayor Rudy Giuliani announced plans in early 1998 to spend $2.6 million on an expansion of the park; the MTA agreed to contribute $400,000 toward the project.

The mezzanine is relatively shallow, and because it was built with insufficient clearance, Union Square Park was raised by 4 feet (1.2 m) to accommodate the station.

An entrance with one stair, escalator bank, and elevator in the Zeckendorf Towers, at the northeast corner of 4th Avenue and 14th Street, connects with the southeast end of the station.

[4]: 18 [164] The Zeckendorf Towers entrance is privately maintained; as of 2023[update], the escalators there had been among the least reliable in the New York City Subway system for several years.

One block to the west, two staircases on the south side of 14th Street between Broadway and University Place lead to the western Canarsie Line mezzanine.

[4]: 18 [165] The central portion of the station contains another exit from the Lexington Avenue Line mezzanine to the Zeckendorf Towers, which leads to the southeast corner of Union Square East and 15th Street.

[174] According to a separate report by engineering firm STV, it would be prohibitively expensive to relocate the southbound platform northward to eliminate the gap fillers.

[4]: 6  The mosaic tiles, by Jay Van Everen, are part of a work entitled The junction of Broadway and Bowery Road, 1828, a reference to the two streets that intersect at Union Square.