She was a member of the Women's Labour League from 1913, a member of the Labour Party, was active in municipal government as a councillor on Kensington Borough Council for 13 years and was one of the first women Justices of the Peace. She stood as the Labour Party candidate for Islington East in the General Elections of 1922, 1923 and 1924. As a senior woman in the Labour Party, she was also a member of the National Executive Committee (NEC) from 1918 - 1920, 1922 - 1926 and 1928 - 1931.
Bentham was finally successful in the 1929 general election, becoming at 68 years of age the oldest woman to be elected as Member of Parliament and she was the 15th ever woman to be elected in Britain. This coincided with the election of the second ever Labour Government headed by Ramsay MacDonald.