Reared in upper New York state, Eastman graduated from Vassar College, Poughkeepsie, New York, in 1903 and from the New York University School of Law in 1907, ranking second in her class. With her influential report “Work Accidents and the Law” (1910), she earned a position on the New York State Commission of Employee's Liability and Causes of Industrial Accidents, Unemployment and Lack of Farm Labor. As a member of the commission, she worked for the enactment of health and safety laws in the workplace and drafted the first worker's compensation law. Eastman also rallied for woman suffrage and founded, with Alice Paul (Paul, Alice), Lucy Burns (Burns, Lucy), and others, the Congressional Union in 1913, which later became the National Woman's Party. Lobbying against U.S. involvement in World War I, she joined the peace movement and led the New York branch of the Women's Peace Party (Woman's Peace Party) throughout the war. In 1917 she cofounded the National Civil Liberties Bureau (NCLB), which reestablished itself in 1920 as the American Civil Liberties Union (ACLU). Although a firm believer in socialism as the most likely way to achieve equality in society, Eastman supported the Equal Rights Amendment of 1923. Throughout the remainder of her life, she contributed writing to feminist journals and continued to fight for equal rights for women.

Her feminist ideals shaped her personal life as well. When she divorced her first husband in 1915, she refused to accept alimony, criticizing the concept as nothing less than an admission of financial dependency on men.