Mary Stack McNiff worked at Brookline Public Library and Harvard, and at St. John's Seminary as Assistant to the Librarian.
Together with her husband, Philip J. McNiff, she was a devoted Catholic. They participated actively in the communities of Boston College and St. John’s Seminary by attending or sponsoring events for visitors. In particular, the McNiffs supported the Boston College Humanities Series, overseen by Father Francis Sweeney. Mary Stack McNiff wrote book reviews for Catholic publications The Pilot and Commonweal, and for the Boston Globe. She also maintained an extensive correspondence with clergy and with authors and artists. McNiff died in 1976.