Stephen was the youngest member of the WSPU Glasgow delegation to the Chancellor of the Exchequer David Lloyd George in 1912, and, she led the first of the "Scottish Outrages", involving attacks on pillar boxes, in Glasgow in February 1913. Her job as a maid worked in her favour during these attacks, as she explained in a 1975 interview:"I was able to drop acid into the postal pillar boxes without being suspected, because I walked down from where I was employed in my cap, muslin apron and black frock... nobody would ever suspect me of dropping acid through the box."Stephen was approached by Sylvia Pankhurst and moved from Glasgow to London, where she became considered one of the "most active members" (along with Emma Boyce, around 1916) of the Workers' Suffrage Federation. In April 1919, Stephen was one of a number of speakers to address a crowd of "about 10,000 people" in Trafalgar Square, opposing the Blockade of Germany. Other speakers included Emmeline Pethick-Lawrence and Theodora Wilson Wilson. She was also an active member of the Women's Peace Crusade and at the 1920 ILP conference argued against the use of force during events preceding the Treaty on the Creation of the USSR.