She declared herself a socialist in 1906 and was an early advocate of better wages and safer working conditions for women. She served on the state commission which drafted the first minimum wage law in the U.S. In 1918, after three years of working with a women's organization that wanted to stop the Great War, she was fired from her teaching job. She had joined with Jane Addams in the effort to stop the war. She and Addams worked for the Women's International League for Peace and Freedom (WILPF), for which Greene Balch served as Secretary General in the Geneva headquarters. It was she who facilitated the U.S. withdrawal from Haiti in 1926, after 11 years of occupation.

In 1915, Greene Balch attended a conference between the Scandinavian and Russian governments in which peaceful resolutions to the Great War were discussed.

Although she was a Quaker and an absolute pacifist, Greene Balch finally decided to support the war against Hitler in 1941. She also supported the war against Japan after Pearl Harbor, although she opposed the U.S. government's demand for unconditional surrender by the Japanese.

In 1946, she received the Nobel Peace Prize, which she shared with American evangelist John Mott (of the YMCA). Nobel Committee chairman Gunnar Jahn commended Greene Balch on her gradualism compared with the Utopianism of less patient peace workers.

Emily Greene Balch retired in 1956 and died in 1961.