Hester Diamond, a New York art collector, art dealer and interior designer who joined with her first husband in amassing an astonishing Modernist collection before tossing it aside in favor of old masters, died on Jan. 23 at her home in Manhattan. She was 91.

Her son David Diamond said the cause was metastatic breast cancer.

Ms. Diamond’s career spanned more than six decades, beginning with a part-time gallery job in the 1950s and culminating in the presidency of a research institute dedicated to Florence’s Medici family.

A self-taught art expert, Ms. Diamond always insisted that her first criterion for a purchase — maybe her only one — was loving the piece.

“There was no strategy,” she said in a 2017 oral history interview for the Archives of American Art and the Center for Collecting in America. “See an opportunity. Find the buyer. See how it goes.”