While there, she worked with Charles B. Davenport at the Cold Spring Harbor Laboratory, becoming so enamored of eugenics that her friends nicknamed her "Eugenia.

Subsequently, Mary and a group of 80 debutantes established the Junior League for the Promotion of Settlement Movements in 1901, while she was still a student at Barnard College.

With better preparation came greater engagement leading to increased interest in membership by women notable in New York society; members would come to include Eleanor Roosevelt, Dorothy Whitney Straight and Ruth Draper.

In time, Leagues would expand their efforts beyond settlement house work to respond to the social, health and educational issues of their respective communities.

Averell was encouraged by his older sister to leave his finance job and join her and their friends, the Roosevelts, to advance the goals of the New Deal.

On May 27, 1910,[9] Mary married sculptor and polo player Charles Cary Rumsey (1879–1922), shortly after the death of her father on September 9, 1909.

[18] Though the exact nature of their relationship is unknown, Perkins was with Rumsey's children at her bedside when she died, and was publicly recognized as one of the 'principal mourners' at her funeral.