Additionally, New York Life completed a series of renovations to the original building during the late 20th century.

The building was designed for the New York Life Insurance Company for three main reasons: to provide expansion space, as an investment, and as an icon.

[11] The New York Life Building stands 615 feet (187 m) tall and contains 34 floors, though is technically 40 stories high.

[15] The New York Life Building combines streamlined Gothic details and a massing that is distinctly Moderne in design.

The lobby is designed similar to the nave of a cathedral,[10][24] with travertine walls, a painted coffer ceiling with barrel vaults 38 feet (12 m) tall, as well as bronze ornamental grilles on doorways and elevators.

[14][24] The eastern side of the first basement level incorporates an entrance to the downtown platform of the New York City Subway's 28th Street station, serving the 6 and <6>​ trains.

[14] Another basement contains a vault developed in conjunction with engineers from the Federal Reserve Bank of New York and the United States Department of the Treasury.

The vault contained features such as a time lock, an 800-pound (360 kg) steel door, and some 80 miles (130 km) of wiring for the alarms.

[14][24] The clerical department was located on the second through fifth floors and contained the largest pneumatic tube system in the United States at the building's completion.

In the original design, the interior spaces were heavily soundproofed with thick glass panes, acoustic ceilings, and forced ventilation.

[27] The space was used extensively for cultural and military events, and Madison Avenue on the park's eastern border became an upscale residential row.

[33][34] The New York Life Insurance Company had been chartered in 1841 and was originally located in the Financial District of lower Manhattan.

[37] New York Life took up the mortgage of MSG in 1912,[16] and bought the facility outright four years later, when the venue went bankrupt.

According to architectural writer Kenneth Gibbs, these buildings allowed each individual company to instill "not only its name but also a favorable impression of its operations" in the general public.

[41][43] Furthermore, life insurance companies of the late 19th and early 20th centuries generally built massive buildings to fit their large clerical and records-keeping staff.

[44]By the 1920s, New York Life was undergoing another period of rapid growth, and operations could no longer fit in the 346 Broadway building.

[34][49] The plans were slightly changed from Gilbert's original: the tower was expanded to seven stories, and the setbacks and light courts were minimized.

[52] In August 1926, a new plan was released for a 34-story limestone structure with setbacks, a pyramidal roof, and a Gothic design inspired by French and Dutch architecture.

[56][9] By then, Gilbert had lost interest in the construction process, and his office was "simply approving or disapproving [...] Starrett Brothers' decisions, largely without comment" by 1927.

[56][9][57] The ceremonial cornerstone, laid in June 1927,[58] was filled with documents such as a copy of The New York Times and various company-related reports.

[60] During that month New York Life transferred 75 million documents representing $6.85 billion in policies to the new building.

[26] The remaining floor area was rented out to other commercial and office tenants, such as wool firms,[65] a drug store,[66] and New York University.

[67] For several years, New York Life retained ownership of the Diana statue, finally shipping it to the Philadelphia Museum of Art in 1932.

[22] The New York Life Insurance Company continued to maintain its headquarters in the building, but started leasing extra office space through Cushman and Wakefield in 2004.

[76] At opening, New York Life president Darwin P. Kingsley described the structure as "a majestic cathedral of insurance".

[10] Miriam Berman, a historian, described the gold-plated roof as one "that catches and reflects the sunlight by day and by night is one of the more easily recognized shapes on the city's illuminated skyline".

"[78][79] George Shepard Chappell, writing in The New Yorker under the pseudonym "T-Square", said that Gilbert had been "allowed the luxury of a definite 'style'" and considered the sparing ornamentation to be "decidedly refreshing".

[9][80] Robert A. M. Stern, in his book New York 1930, said that even as the building "was remarkably simplified by comparison with Gilbert's previous works, it was also more stolid".

[81] The National Park Service said that 51 Madison Avenue was "an excellently maintained example of Cass Gilbert's work" whose plans "best represents the large, well-structured organization of the New York Life Insurance Company" in its heyday.

"[84] Charles Phelps Cushing wrote that the gilded roof resembled an "inverted ice cream cone, of golden brown pastry, stamped in the Nabisco pattern".