Mary Wilhelmine Williams was born on a farm in Stanislaus County, California, to parents who had emigrated from Denmark and Sweden. The family was poor, and Mary Wilhelmine had to leave school after eighth grade for a period of time. At age 18, she enrolled in San Jose State Normal School, graduating in 1901. She taught for three years before entering Stanford University, where she received her M.A. in 1908. Intent on becoming an historian, she returned to teaching to support herself while studying at the University of Chicago during the summers, and finally received her PhD in 1914. She joined the faculty at Stanford as an instructor in history and later held appointments at Wellesley College and Goucher College, where she was named a professor in 1920.
She specialized in Latin American history. In 1918-1919, Prof. Williams worked for the government of Honduras as a cartographic, geographic, and historical adviser in its border disputes with Guatemala and Nicaragua. The U.S. State Department appointed her to a variety of committees dealing with Latin American problems. She contributed to the Dictionary of American Biography and wrote numerous scholarly articles. Her books included The People and Politics of Latin America (1930), a general survey that remained a standard text for college students for nearly 20 years, and two books on Scandinavian history, as well as a biography, Dom Pedro the Magnanimous: Second Emperor of Brazil (1937). Prof. Williams was an active feminist and pacifist, a member of the Women's International League for Peace and Freedom, and a founder of the California chapter of the National Woman's Party. She also served as the editor of Equal Rights, an independent feminist weekly, from 1935 to 1936.