The William Adams Brown Ecumenical Library (WAB) was founded by the Union Theological Seminary (UTS) Board of Directors as a response to the increasing demand for materials in the field of ecumenicity. The Library was named after William Adams Brown, who was very active in the field of Christian Unity and was Roosevelt Professor of Systematic Theology at UTS. Upon his death, he left his sizeable collection of papers documenting his activities with ecumenical conferences and committees, which served as the basis for the Library. In November 1944, the Library was opened, and was directed by a committee of faculty in collaboration with the Missionary Research Library (MRL). The funding for WAB, as well as for part-time staff, was from UTS. The official opening of the Library occurred on March 13, 1945. The general policy of the Library was to acquire and make available printed and unprinted material documenting the ecumenical effort, including records connected to church union, federation and cooperation projects, and the development of the World Council of Churches. WAB, as well as MRL, later became part of the UTS library.

Henry Pitney Van Dusen was a faculty member of UTS, as well as Union's president from 1945-1963. Van Dusen was very supportive of the library and did all possible in order to keep it functioning. He was also one of the main architects of the World Council of Churches, and is among the most important ecumenists of the twentieth century.