The mansion is a New York City designated landmark and, along with the Burden House, is listed on the National Register of Historic Places.

The house has French- and Italian-style interiors and is divided into front and rear sections, with an oval entrance hall connecting the floors.

Andrew Carnegie purchased the site in 1898 to protect the value of his nearby mansion, but he did not sell it until 1913, when Kahn bought the plot.

After the house was completed, the Kahns hosted various events there, splitting their time between their 91st Street residence and Oheka Castle on Long Island.

After Otto Kahn died in 1934, the Convent of the Sacred Heart bought it and converted the house into classrooms, a library, and offices.

[13] The townhouses on 11, 15, and 17 East 90th Street and the Spence School are located on the same block as the Carnegie Mansion, southeast of the Kahn House.

[14] The architects J. Armstrong Stenhouse and C. P. H. Gilbert designed the house at 1 East 91st Street for the family of financier Otto Hermann Kahn in the neo-Italian Renaissance style.

[25] The second story is designed like a piano nobile and is rusticated,[25] albeit with smoother limestone and shallower joints compared with the first floor.

[18] Visible from the roof was a small garden house, a fountain, and a porch, which one critic from 1919 described as "a glimpse of Italy, in more intimate guise".

[27] A courtyard is on the northern side of the site and extends into the center of the building, allowing rooms in the middle of the house to be illuminated by natural light.

[31] At the rear of the house's northeast corner is an annex for the Convent of the Sacred Heart's lower school, which has a limestone facade.

[36][34] There was originally a garden and private driveway, which was guarded 24 hours a day by a doorman,[37] as well as an oak-paneled library and spacious reception room.

A hexagonal shaft, with a spiral staircase, connects Otto Kahn's former third-floor bedroom and his fourth-floor study (the latter of which serves as a library for the school).

[19] The Convent of the Sacred Heart's lower-school annex, at the northeast corner of the house, includes a library, classrooms, and offices.

[47] In December 1898, the industrialist Andrew Carnegie bought all of the lots on Fifth Avenue between 90th and 92nd streets, with the intent of building his mansion on some of these plots.

[51][54][53] Carnegie tried to split up the parcel at the northeast corner of 91st Street and Fifth Avenue in 1906, with plans to sell half of it to the politician Lloyd S.

[67] After the Sloanes dropped their lawsuit against Carnegie,[58] Kahn sold a 26-inch-wide (66 cm) strip of land on the eastern side of the lot to the Burden family.

[79] He, his wife Adelaide (Addie), and their children Maud, Margaret, Gilbert, and Roger mainly lived at the 91st Street mansion afterward.

[83] The house was decorated with tapestries, in addition to paintings from Kahn's art collection, such as Vittore Carpaccio's late-15th-century portrait St.

[95] Kahn reportedly lost over $50 million during the Wall Street Crash of 1929, although the family retained ownership of the house.

[98] The Oheka Corporation, representing Kahn's estate, filed a lawsuit in May 1934 to have the restriction removed so they could sell the house to the Convent of the Sacred Heart.

[104] In May 1934, the Convent of the Sacred Heart paid $900,000 for the house and sold its existing building on Madison Avenue to the Oheka Corporation.

[44] One critic called the sale a sign of the transformation of the surrounding neighborhood,[19] while the real-estate executive Lawrence B. Elliman described the Kahn House as one of the largest "disappearing New York mansions" that were being converted to non-residential uses.

[45] There were also occasional tours of the Kahn House,[40] and the structure continued to host events into the late 20th century, including TV commercial shoots[118] and antiques shows.

[34] The architectural firm of Buttrick, White, and Burtis was hired in the early 1980s to add a gymnasium, laboratories, and classroom space in the Burden and Kahn houses.

The project, which was estimated to cost $1.5 million, was funded in part from revenue generated by the Burden House's ballroom,[120] as well as from events such as tours and Christmas tree sales.

[36][121] Building Conservation Associates also examined the Kahn House's facade in the 1990s and found that it had been damaged over the years by the acidity of the air.

As such, Buttrick, White, and Burtis also supervised a restoration of the facade at a cost of $1.3 million; the project included cleaning the stone and infilling some of the larger holes.

[15] Jayne Merkel wrote in 1996 that the Kahn and Burden houses "give the school a uniqueness and elegance that money cannot easily buy".

[46] The house was used to film television commercials in the late 20th century, as well as a video for Julio Iglesias's song "Moonlight Lady" in 1984.