Heloise Brainerd was born in Wallingford, Vermont on April 30, 1881 to Charles and Emily (Sanford) Brainerd. She attended Middlebury College in 1900-1901, and graduated from Smith College in 1904. After graduation, she worked from 1905-1909 as a bi-lingual secretary in a law firm in Mexico City, living part of the time with Mexican families. From 1909-1935 she was connected with the Pan American Union in Washington, D.C., first as private secretary to the Assistant Director, and then as Chief of the Division of Education; this later expanded into the Division of Intellectual Cooperation, which worked to foster educational, artistic, literary, scientific and cultural interchange in the Americas. Brainerd spent six months in South America in 1928 as the official representative of the Pan American Union, during which time she visited Panama and all of the republics except Colombia and Venezuela.
In 1935 Brainerd resigned from the Pan American Union and became the chair of the Committee on the Americas, and chair of the Division of Inter-American Work, for the Women's International League for Peace and Freedom. Brainerd's goal was to draw women from Latin America into the peace movement, and to do this she traveled widely in the Americas, often staying for lengthy visits, and corresponded frequently with women there [see Committee on the Americas, WILPF Collection (DG 043)]. She resigned from the WILPF in 1953.
Brainerd was made Honorary Vice President of the U.S. Section of the WILPF in 1954. Other honors received included the Medal of Public Instruction by Venezuela and the Order of Merit by Ecuador, as well as being made an honorary member by numerous organizations.
Brainerd died on February 16, 1969.