Stewart-Brown became Honorary Secretary of the ladies' branch of the Liverpool Royal Society for the Prevention of Cruelty to Animals. She joined the Women's Liberal Federation in Liverpool and helped form branches at West Derby and Wavertree and served as President for the West Toxteth branch. In 1891 she began speaking for women's suffrage. She was the first woman to speak on a municipal platform when her husband stood for Liverpool City Council in 1892. In the same year she was elected to the national executive committee of the Women's Liberal Federation, a body on which she served for many years. In 1894 she co-founded the Liverpool Women's Suffrage Society with Mrs Alfred Booth and Edith Bright, at a meeting at which Millicent Fawcett was the guest speaker. She became its branch chairman. She became President of the Liverpool Women's Liberal Association and President of the Lancahire & Cheshire Union of the Women's Liberal Association. In October 1911, with Eleanor Rathbone she founded a new society, the Municipal Women's Association, aimed at 'awakening...interest on the question of women's suffrage amongst women municipal voters'. As a committed Liberal and Suffragist, she opposed the actions of the militant Suffragettes. In 1912 she opposed the establishment of the NUWSS's election fighting fund which was intended for campaigning against Liberal candidates in by-elections and attempts to align the NUWSS with the Labour Party. Following the outbreak of war in 1914 she switched her focus to relief work, helping the Scottish Women's Hospitals' Association and the Liverpool branch of the National Union of Women Workers. After the war she became Liverpool branch Chairman of the Women's International League and was on the executive of the Liverpool branch of the League of Nations Union. In 1919, standing as a Liberal candidate, she became only the second woman to be elected to Liverpool City Council. She was Liberal candidate for the safe Unionist seat of Waterloo, a division of Lancashire at the 1922 General Election. She did not stand for parliament again. She became President of the Waterloo Women's Liberal Association. In 1924 she became one of the first women appointed a Justice of the peace, serving on the Liverpool bench. She was President of the Liverpool National Council of Women, Vice-Chairman of the Council of Women Citizens and Chairman of the Liverpool Liberal Council.
