Estelle Sylvia Pankhurst (she later dropped her first forename) was born at Drayton Terrace, Old Trafford, Manchester, a daughter of Dr Richard Pankhurst and Emmeline Pankhurst, who both later became founding members of the Independent Labour Party and were much concerned with women's rights. Sylvia and her sisters, Christabel and Adela, attended Manchester High School for Girls, and all three became suffragists.
In 1906, Sylvia Pankhurst started to work full-time for the Women's Social and Political Union (WSPU) with her sister Christabel and their mother. She applied her artistic talents on behalf of the WSPU, devising its logo and various leaflets, banners, and posters as well as the decoration of its meeting halls. In 1907 she toured industrial towns in England and Scotland, painting portraits of working-class women in their working environments. She spent time in Leicester where she was welcomed by Alice Hawkins who she knew through the Independent Labour Party. They were soon joined by Mary Gawthorpe and they established a WSPU presence in Leicester.
In contrast to Emmeline and Christabel, Sylvia retained an affiliation with the labour movement and concentrated her activity on local campaigning. She and Amy Bull founded the East London Federation of the WSPU. Sylvia also contributed articles to the WSPU's newspaper, Votes for Women and, in 1911, she published a propagandist history of the WSPU's campaign, The Suffragette: The History of the Women's Militant Suffrage Movement.
By 1914, Sylvia had many disagreements with the route the WSPU was taking. It had become independent of any political party, but she wanted it to become an explicitly socialist organisation tackling wider issues than women's suffrage, and aligned with the Independent Labour Party. She had a close personal relationship with the Labour politician Keir Hardie. On 1 November 1913, Pankhurst showed her support in the Dublin Lockout and spoke at a meeting in London. The members of the WSPU, particularly her sister Christabel, did not agree with her actions, and consequently expelled her from the union. Her expulsion led to her founding of the East London Federation of Suffragettes, which over the years evolved politically and changed its name accordingly, first to the Women's Suffrage Federation and then to the Workers' Socialist Federation. She founded the newspaper of the WSF, Women's Dreadnought, which subsequently became the Workers' Dreadnought. The federation campaigned against the First World War and some of its members hid conscientious objectors from the police.