Over the first half of the 20th century, the house was successively the residence of businessmen Isaac D. Fletcher and Harry F. Sinclair, and then the descendants of Peter Stuyvesant, the last Director of New Netherland.

The mansion was designed in an eclectic French Renaissance style by C. P. H. Gilbert and built by foreman Harvey Murdock.

The northern facade on 79th Street, containing the main entrance, is characterized by multiple windows in square recesses or semi-elliptical and fully Gothic arches.

[2] The building is surrounded by a lawn, sunk into the ground,[3][5] that is itself enclosed by a wrought iron fence, broken only by a stair and balustrade approaching the main entrance, on the north side.

[8][11] Cook intended the block to house first-class residences, not high-rises, and only sold lots for the construction of private dwellings.

[16][3] After taking ownership of the house, the Fletcher family moved their large art collection there, including paintings by Jacques-Louis David, Thomas Gainsborough, Rembrandt van Rijn, Joshua Reynolds, and Peter Paul Rubens.

[33][34] The UIA's purchase of the Sinclair House gave the structure a "temporary reprieve" from demolition, as described by Newsday; at the time, several other mansions on Fifth Avenue were being demolished.

[22] In 1977, the New York City Landmarks Preservation Commission designated the house as part of the Metropolitan Museum Historic District,[16] a collection of 19th- and early 20th-century mansions around Fifth Avenue between 78th and 86th Streets.

[39] The UIA began repair work on the roof of the Sinclair House in late 1996 at an estimated cost of $250,000 (equivalent to $485,678 in 2023).

[22][1] In an interview with The New York Times that year, a member of the board described this work as an interim measure, as the building was in a poor state.

[40] In November 2003, the US government made a matched grant of $270,000 (equivalent to $447,200 in 2023) to the UIA through the Save America's Treasures initiative to cover the costs of modernizing the building's electrical wiring and plumbing.

[41] The state government's Office of Parks, Recreation and Historic Preservation granted the UIA another $70,000 (equivalent to $112,918 in 2023) for restoration in June 2004.

[42] By July 2009, the UIA had completed improvements to the electrical wiring, installed a security system, replaced windows, and restored design elements.

[44] The foreman, Harvey Murdock, was also prolific both in the construction of private residences in Manhattan and Brooklyn, and had worked with Gilbert several times prior to the Sinclair House.

The main entrance is a frontispiece, a staple of French Renaissance homes, placed just to the left of the facade's center.

[46] The western elevation of the facade is symmetrical and dominated by a curved, projecting pavilion, rising from the basement to the cornice.

Belt courses run along the entire facade, separating the floors and terminating at the corners with sculpted gargoyle heads.

The eastern part of the house is used as a coat room, an office, and a service entrance leading to the secondary stairway.

[6] The top two floors, within the mansard roof,[46] have been transformed from servants' quarters into office space for the UIA's staff.

[27][6] An 1899 article in the Real Estate Record and Guide generally praised the composition of the Sinclair House but noted that it had a rather ecclesiastical appearance and did not much resemble other, then-contemporary New York manors.

[53] The 2010 AIA Guide to New York City characterized the house as "a miniature French-Gothic chateau squeezed into the urban context".

"[54] He praised the "whimsical details", including what he described as "the carved dragon fish in the railings and those figures in funny hats holding up the windows".