"The Main Lobby, the principal point of entry, is aproached from Fifth Avenue by a handsome flight of steps leading up to a wide triple-arched portico. Three paired pedimented entrance doors, set beneath high arched transoms of glass, are embellished by ornamental wrought-iron borders. The great interior space of the entrance lobby is over seventy feet long and forty-four feet wide, rising to a height of about thirty-four feet. This huge space is all of stone, including the two grand staircases leading to the upper floors. To achieve this monumental effect, the architects introduced a low-arched barrel vault of stone with lateral penetrations from the side arches"

Apparently, the Main Branch has been given several historic designations. It was declared a National Historic Landmark in 1965, listed on the National Register of Historic Places in 1966, and designated a New York City designated landmark in 1967.  The main reading room was separately made a New York City designated landmark in 2017.

Landmarks Preservation Commission describes the Central Hall pointing out that, "... except for the colored marble base and the door enframements on two sides, is of dark wood up to the high cornice line from which springs a paneled barrel vault of plaster. The side walls consist of a series of arched openings in shallow bays designed for murals. The arches, separated by paired fluted pilasters with Corinthian capitals, support the handsome entablature which is crowned by a richly molded cornice resting on evenly spaced console brackets. An unusual feature of this rectangular room is the manner in which the barrel vaulted ceiling is carried out over the cornice line at the north and south ends to receive light from segmental-arched windows designed to stress the diagonal.  The barrel vault has three large panels on each side, crowned at its apex by a long rectangular enframement which is curved at each end. It surrounds an aerial, illusionistic painting which gives the effect of an opening to the sky.  The murals in this room were painted by Edward Laning."