Educated at Heathfield in England and the Sorbonne in Paris, Mary Slack returned to Johannesburg and started a long association with the Ernest Oppenheimer Memorial Trust, a foundation established in memory of her grandfather. Now re-named the Oppenheimer Memorial Trust, and still supporting educational projects and social development, Mary now heads up the organization.
Besides collecting artworks, Mary also became involved in the arts, first serving on the boards of Dorkay House and the African Music and Drama Academy, later on that of the Federated Union of Black Arts. She was Managing Trustee of the Market Theatre from 1990-1993 and was instrumental in raising the profile of the arts among the business community. In 1997 she took the Chair of Business and Arts South Africa, a joint initiative of government and business to secure the development of the arts in South Africa.
Currently, Mary holds positions in many organizations including the Mali Manuscripts Trust and the Paleo-Anthropological Scientific Trust. She is also a Trustee of the Helen Suzman Foundation.
During the 1990s, Harry Oppenheimer invited her to attend meetings of the board running The Brenthurst Library and since his death in 2000 she has been responsible for both the Library and The Brenthurst Press.