The wife of Edward Harkness, she participated in his philanthropy and also gave independently to various institutions, particularly for women's education, continuing to do so after his death in 1940.

[2][3] Her mother, Elizabeth, was the only surviving child of Thomas Stillman Greenman, a shipbuilder in Mystic, Connecticut.

In 1904, she married Edward Harkness, the wealthy heir of an early investor in Standard Oil.

[4][6] She also donated in 1930 to the then women-only Lady Margaret Hall, Oxford;[7] her gift, made through the auspices of the musician Margaret Deneke when she was touring the United States, was used to build the Deneke Building, designed by Giles Gilbert Scott and completed in 1932.

[10] In 1929, Mary Harkness helped found the Marine Historical Association—now Mystic Seaport Museum—on land that had belonged to her shipbuilding grandfather; in 1945, she would donate his house to the museum as well.