Mary Gallagher was born and grew up in Boston, the youngest of 12 children. She first worked for a local paper company and then through Kenny O'Donnell, a former salesman for the company, she was hired by newly-elected Senator John F. Kennedy to be a secretary in his office, and went to Washington in January 1953. Gallagher worked for Senator Kennedy for a number of years until her first child was born (after marrying Raymond A. Gallagher in 1955). Thereupon, she became a part-time secretary to Mrs. Hugh Auchincloss, and finally became Mrs. Jacqueline Kennedy's personal secretary in 1957, a post she held until 1964.