Mary Mahoney was born in Dorchester, Massachusetts on 7th May, 1845. At eighteen Mahoney found work at the New England Hospital for Women and Children. For the next fifteen years she was employed as a cook and cleaner and it was not until 1878 that she was accepted as a student nurse. Training was rigorous and of the forty-two students accepted in 1878, only four, including Mahoney, graduated.

Mahoney developed a reputation as an outstanding nurse and was asked to look after private patients in Massachusetts, New Jersey, Washington and North Carolina. Her successful career played an important role in overcoming the considerable racial prejudice against African American nurses that existed at this time.

Mahoney became one of the first African-American women to join the American Nurses Association. In an attempt to combat racial discrimination in nursing, Mahoney joined with Martha Franklin and Adah Thoms to establish the National Association of Colored Graduate Nurses (NACGN). A deeply religious woman, Mahoney became the NACGN national chaplain.

In 1911 Mahoney moved to New York where she took charge of the Howard Orphan Asylum for Black Children in Kings Park, Long Island.

Mahoney was a strong supporter of women's suffrage and in 1921, aged seventy-six, was one of the first women in Boston to register to vote after the ratification of the Nineteenth Amendment. Mary Mahoney died of breast cancer on 4th January, 1926, and is buried in Woodlawn Cemetery in Everett, Massachusetts.