Following her husband's death, Diamond switched her focus to Old Masters, assembling "one of the greatest, most idiosyncratic art collections in America".

[4] An only child, she attended Hunter College High School, and often visited the Museum of Modern Art—her favorite museum—and the Guggenheim.

"[6] Becoming renowned as dealers as well as collectors,[7] the Diamonds' personal collection included works by Picasso, Fernand Léger, Mondrian, Constantin Brâncuși, Mark Rothko, and Willem de Kooning.

To finance her acquisitions, she sold significant pieces from her collection, including works by Henri Matisse, Picasso and Wassily Kandinsky.

Her love of the Old Masters inspired her in 1995 to co-found the non-profit organizations the Medici Archive Project, which funds research for students and scholars on Renaissance and Baroque art, and in 2013, a publishing project for scholarship on Old Master sculpture, VISTAS (Virtual Images of Sculpture in Time and Space).

She had one stepdaughter, three stepsons, and three sons: David, Mike, a founding member of the Beastie Boys, and Stephen, who died of neuroendocrine cancer in 1999.