[1] After her parents' separation, the light-skinned Belle, her mother, and her siblings passed as white and changed their surname to Greene to distance themselves from their father.

[9] Belle also made a change to her name, swapping out Marion for "da Costa", and claiming a Portuguese background to explain her darker complexion.

[9] The changes to her and her family's stated ancestry resulted in further fabrications, including one that led people to believe Greene had been raised in Virginia.

[7][nb 1] The true nature of her background was further complicated by Greene's claim to be younger than she was, an action biographer Heidi Ardizzone describes as "a masquerade" in response to a youth-focused society in which "single women past a certain age were disdained".

[1]: 14 Greene began working in the administrative offices at Columbia University's Teachers College sometime in the mid-1890s, where she was introduced to philanthropist and social welfare advocate Grace Hoadley Dodge.

[10][8] Trusted for her expertise–Greene was an expert in illuminated manuscripts–as well as her prowess in bargaining with dealers, Greene spent millions of dollars buying and selling rare manuscripts, books, and art for Morgan.

She told Morgan, who was willing to pay any price for important works, that her goal was to make his library "pre-eminent, especially for incunabula, manuscripts, bindings, and the classics".

[15] In a 1912 profile of Greene, the New York Times referred to her "force of persuasion and intelligence" and recounted her pre-auction purchase of seventeen highly sought after William Caxton books on behalf of the Morgan library.

She was quite successful in this; for instance, when the Morgan Library became a public institution and was named its first director in 1924, she celebrated by mounting a series of exhibitions, one of which drew a record 170,000 people.

One year she managed, by artfully letting the customs agents find several dutiable items of hers in her luggage, to draw their attention away from a painting, three bronzes, and a very expensive watch he had asked her to buy in London.

[6][20] Morgan left her fifty thousand dollars in his will, enough capital for her to live on comfortably, though she continued to supplement her inheritance with the $10,000-a-year salary that she earned at the library, a huge sum at the time, especially for a woman.

Her mother, Genevieve, lived with her for decades, and Greene played an active role in raising her nephew Robert Mackenzie Leveridge, who had been born in her home.

[27] The Medieval Academy of America established the Belle da Costa Greene Award to fund research by medievalists of colour.