Born in Bridgewater, Mass, she was the daughter of Irish immigrants John McCormack and Bridget T./Theresa Kane. Mary never married but she adopted two children, George Armstrong and Margaret McCormack (the latter who she adopted with her brother, Fred McRae).
Mary was a retired stitcher. She also made candy and bread as well. She was formerly employed by the Diamond Shoe Co.
For most of her life, Mary resided at 20 Montauk Road, Brockton. She took in many people over the years, relatives as well as non-relatives.