Considered to be New York City's and the world's first modern office building, the Equitable Life Assurance Building at 120 Broadway was constructed in 1868–1870 to the designs of architects Arthur Gilman and Edward H. Kendall, with George B. Post as consulting engineer. The brick, granite, and iron structure occupied the entire block bounded by Broadway, Cedar Street, Nassau Street, and Pine Street. It featured electric lighting and some of the first elevators in an office building, which allowed the company to lease the floors above their own to commercial tenants. Originally built with seven stories above two underground levels, the structure was expanded in 1885 to nine stories for a total height of 155 feet.

Despite being touted as fireproof, a fire began in one of the building's basements on the morning of January 9, 1912. Open stairways and elevator shafts allowed the blaze to spread quickly. Brooklyn firefighters joined overwhelmed Manhattan crews, but their combined efforts were hampered by the building's height and by the extreme cold: the outside temperature was just 18 degrees Fahrenheit, causing water from their hoses to freeze instantly, caking everything in ice. The blaze was brought under control by 9:30 that evening, but smoke continued to billow from the ruins for two days. There were six casualties—one firefighter, and five Equitable Life employees—but the money in the vaults (appraised at $385 million) was untouched.

The company replaced their destroyed headquarters with a new Equitable Building on the same site, which, on completion in 1915, was the largest office building in the world by floor area. Its great massing (1.2 million square feet of floor space) and height (38 stories) blocked sunlight and air from reaching the ground. This contributed in large part to the adoption of the 1916 Zoning Resolution to limit these drawbacks in future construction.