Born in Boston into a well-off family, she was amongst the first graduates of Bryn Mawr College in 1889. She continued to study sociology and economics in Europe and the US, and in 1896 joined the faculty of Wellesley College, becoming a full professor of economics and sociology there in 1913.
During the First World War, she helped to found the League, and campaigned against America's entry into the conflict.
Her contract terminated by Wellesley because of her pacifist activities, she became an editor of The Nation, a well-known liberal news magazine, acted as secretary of the WILPF (a second term in 1934 without salary for a year and a half), did much work for the League of Nations.
Balch became a Quaker in 1920. She never married.