She was the granddaughter of Henry Morrison Flagler, one of the founders of Standard Oil, and inherited 20,000 shares of Standard Oil from her father Harry Harkness Flagler on his death in 1952 (valued then at $1,600,000). She had married Melbert Cary, who died in 1941.
Mrs Cary and her husband were significant collectors. Their main acquisitions were music manuscripts, playing cards, and the work of great printers. Mrs Cary also built a collection of trees on her large estate at Millbrook, New York (now the Mary Flagler Cary Arboretum).
The Mary Flagler Cary Charitable Trust was established at her death. It has donated major collections as follows:
- The Melbert B. Cary, Jr. Graphic Arts Collection at the Wallace Library Rochester Institute of Technology, Rochester, New York.
- The Mary Flagler Cary Music Collection at the Morgan Library, New York City.
- The Cary Collection of Playing Cards at the Beinecke Rare Book and Manuscript Library, Yale University, New Haven, Connecticut.
The Trust wound up its remaining work in 2009.
- Flagler Museum founded by Mary's Sister, Jean Flagler Matthews.
- Cary Graphic Arts Collection
- Cary Institute of Ecosystem Studies