Doremus aided in founding, in 1850, the House and School of Industry for Poor Women, becoming its president in 1867, and in 1854 became vice-president of the Nursery and Child's Hospital. In 1855 she assisted J. Marion Sims in his project of establishing the New York Woman's Hospital, of which she was ultimately president. During the American Civil War, she cooperated with the work carried on in the hospitals, ministering alike to the wounded from north and south. She founded, in 1860, the Woman's Union Missionary Society, designed to elevate and Christianize the women of heathen lands, and she took an active part as manager in the Presbyterian home for aged women, organized in 1866. She aided in collecting supplies to relieve the sufferers from famine in Ireland in 1869, and was for many years manager of the female branch of the City Mission and Tract Society and of the Female Bible Society. The last society in which she labored was known as the "Gould Memorial," and had for its objects the establishment of Italian-American schools. All foreign missions, without regard to creed, shared her sympathies.