The Carnegie Corporation gave the house and property to the Smithsonian in 1972, and the Cooper-Hewitt Museum opened there in 1976 following renovations by Hardy Holzman Pfeiffer Associates.

[12] Built for the industrialist Andrew Carnegie and later converted into the Cooper-Hewitt Museum,[13] the mansion was finished in 1902[14][15] and was near the north end of Fifth Avenue's Millionaires' Row.

[33] At the southeast corner of the main mansion is the McAlpin–Miller House at 9 East 90th Street, formerly owned by George L. McAlpin and then by Carnegie's daughter Margaret Miller.

[18] The Chicago Daily Tribune wrote that, during the mansion's construction, the structure was variously described as Dutch Colonial Revival and French Renaissance, although it incorporated elements of several architectural styles.

[56] The southern elevation of the house is largely built of brick with stone trim, except the first story, which is made of rusticated blocks of white marble.

[85] At the eastern end of the main hall was a 3,000-pipe organ,[97][98] which was played regularly until 1946 and was moved to the Nassau Center for the Fine Arts in Roslyn Harbor, New York, in 1974.

[90] Near the southeast corner of the house is the breakfast room, which could fit 22 people; it was originally outfitted with walnut paneling, bronze-and-glass lamps, a plaster ceiling with molded geometric patterns, and a custom wall covering.

[74][76] Originally, there was a stuffed barracuda at the top of the stairway landing, as well as a central hall with columns, painted pilasters, and oak ceiling beams.

[91] De Forest designed the family library, which had an ornate fireplace, frieze, paneling, corbels, and stenciled ceiling decoration; it was illuminated by Tiffany lamps.

[129] The fourth story consisted of numerous bedrooms arranged around a hall;[91] at the center of the hallway, a bronze railing surrounded the third-floor laylight.

[130][131] Carnegie had shared a hotel suite with his mother until he married Louise Whitfield at the age of 51;[132] they then lived near Fifth Avenue and 51st Street in Midtown Manhattan.

[138] After the initial blueprints were complete, Carnegie invited Henry J. Hardenbergh; Howard, Cauldwell & Morgan; and Babb, Cook & Willard to prepare plans for the mansion.

Carnegie typically spent his mornings working in the library and exercising; after an afternoon nap and a walk around Central Park, he hosted business visitors.

[106] Carnegie began allowing local children to play in the mansion's garden in 1911,[202] and Louise's brother Henry D. Whitfield designed a passageway between the house's conservatory and picture gallery in 1913.

[219] Events included the 1927 marriage of the Carnegies' niece Louise Whitfield,[220] as well as sewing classes,[221] student club meetings,[222] parties in the mansion's garden,[223] and concerts.

[228][229] Following Louise Carnegie's death, the mansion remained largely intact, and it was maintained by the family steward Alexander Morrison and a skeleton crew.

[237] The writer Heather Ewing stated that the house and garden were "a fortress of peace from the outside realities" and helped inspire the School of Social Work's students.

[78] In addition, Taylor and renovation architect Hugh Hardy planned to convert the mansion's basements into exhibit space once the museum had raised more money.

[278] Paul W. Thompson, who succeeded Pilgrim in 2000, initially did not plan to expand the mansion and townhouses,[279] but he changed his mind after the museum experienced staffing, budgetary, and exhibit shortages.

[278] The Target National Design Education Center, comprising a library, studio, and lecture room, opened on the mansion's ground floor in 2006.

[282][283] News media reported in February 2005 that the Cooper-Hewitt was considering a $75 million proposal by Beyer Blinder Belle to construct three basement stories under the mansion's garden, thereby nearly quadrupling gallery space to 30,000 square feet (2,800 m2).

The $25 million plan entailed moving the museum's offices and library to the 90th Street townhouses and expanding the gallery space in the mansion itself to 18,000 square feet (1,700 m2).

[293] In addition to the new gallery and relocated offices, the project involved restoring architectural details and adding a freight entrance, a cafe, an enlarged gift shop, and restrooms.

[301] The Buffalo Enquirer wrote that, despite the paucity of exterior detail, the interior was as elaborate as the William A. Clark House, which itself was compared to the Metropolitan Museum of Art's building.

[19] When the mansion was completed, The New York Times compared it with the lavish houses of William Clark, Charles Yerkes, and George Crocker, all located further south on Fifth Avenue.

[307] Wolf Von Eckardt, a critic for the Washington Post, wrote in 1973 that the School of Social Work's renovations had made "the place look as much as a slum as possible".

[75] In 1981, a writer for the Christian Science Monitor described the contrast between the house's use of heavy oak wood, its "delicate architectural details in smaller rooms, and green plants in the conservatory".

[83] Herbert Muschamp wrote that the mansion was "monument to Andrew Carnegie's taste for dark, carved wood" rather than an appropriate setting for the Cooper-Hewitt Museum.

[274] In part because Carnegie had bought the surrounding lots and resold them only to people who would build similar mansions, the historian Christopher Gray wrote in 2014 that the house helped form one of the "grandest blocks" in the city.

A. Haggin, William B. Leeds, Daniel G. Reid, and Henry Phipps Jr. also built their mansions on the northern section of Fifth Avenue after Carnegie's house was completed.