She became dedicated to philanthropy, donating the land that became Harriman State Park and largely funding the development of the controversial Eugenics Record Office.

She was tutored at home and completed her education at a finishing school with the "expectation that one day she would become a fine wife and mother for some young man of equal or greater social standing than the Averells."

Mary's father, William J. Averell, was a successful New York banker and president of the Ogdensburg and Lake Champlain Railroad.

When, in 1909, the state of New York acquired a parcel of land at Bear Mountain to build a new prison, Harriman approached New York Governor Charles Evans Hughes with a proposal to extend the Palisades Interstate Park with a donation of thousands of acres and one million dollars as an endowment for its management if the governor would agree to locate the prison somewhere else.

Monies were contributed to The Boys' Club of New York that E.H. loved and supported, to the American Red Cross, to John Muir to help save the Yosemite Valley and to Yale University for an endowed chair of Forestry.

But many today[weasel words] would regard it as a serious blot on her reputation that she gave over a half-million dollars to the Eugenics Record Office, upon the encouragement of her friend David Starr Jordan.