While in Chicago, Jack led efforts and rallies to prevent the United States' entry into World War II and fought racial segregation.
[1] He was the executive secretary of Chicago Council Against Racial and Religious Discrimination from 1943 to 1948, and from 1948 to 1959 served as the minister of the Unitarian Church of Evanston, Illinois.
Jack co-founded and was the associate director of the American Committee on Africa from 1959 to 1960, co-founded and served as executive director to the Congress of Racial Equality and National Committee for a Sane Nuclear Policy (SANE) from 1960 to 1964, and directed the Social Responsibility Department of the Unitarian Universalist Association in Boston from 1964 to 1970.
Simultaneously, from 1973 to 1984, he chaired the NGO Committee on Disarmament, Peace and Security at the United Nations Headquarters.
[1] In the late 1980s, Jack retired from official positions and moved to Swarthmore, Pennsylvania, where he authored two books and continued to be active in various peace and human rights organizations.