In 1922, Cushman moved to Angola, West Africa,[7] to serve as a medical missionary by the American Board of Commissioners for Foreign Missions.
She served most of the general population of Angola, taking far trips for extreme cases, and having many sick people travel to her for treatment.
One of her notable acts as a missionary was working on staffing the hospital solely with the indigenous people of the area that she had trained.
In 1944, Cushman published a book she had written about her experiences called Missionary Doctor: The Story of Twenty Years in Africa.
The Trinitarian Congregational Parish in Castine, Maine, maintains the Dr. Mary Cushman Circle to raise funds to continue to support missionary service locally and internationally.