A frequent presence on best-dressed lists, Ryan died at a hospice of complications of lung cancer, her son, Drew Dixon Ryan, told The New York Times.
The Bristol, R.I., native was credited with encouraging her friend, cabaret singer Kay Thompson, to write a book about Eloise, a character Thompson played to amuse her friends.
She introduced Thompson to illustrator Hilary Knight, the woman whose drawings would capture young Eloise's adventures at Manhattan's posh Plaza Hotel in the 1955 book, "Eloise," as well as several popular sequels.
"The book would never have existed without D.D. Never," Knight told the Times.
Ryan is also known for working as a "Harper's" photo editor under renowned editor Diana Vreeland, designing the costumes for Stephen Sondheim's 1970 Broadway musical "Company" and hanging out with the likes of Cole Porter, Andy Warhol and Truman Capote.
Ryan is survived by her sons, Drew Dixon Ryan and Beau Barry Ryan, and three grandchildren.
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