On her return to England she joined the National Union of Suffrage Societies and the Central Society for Women's Suffrage. In 1907 she joined the Women Social and Political Union and was arrested during a demonstration outside the House of Commons. However, she was released without charge. In an article she published in the Votes for Women she argued she was willing to "follow these women to prison or to death".
In February 1908 she was arrested for taking part in another demonstration. She was sentenced to six weeks' imprisonment in Holloway Prison. On her release she became a regular speaker at WSPU meetings. Emily Blathwayt claimed that she was "one of their (WSPU) best London speakers."
The WSPU organised a mass meeting to take place on 21 June 1908 called Women's Sunday at Hyde Park. The leadership intended it "would out-rival any of the great franchise demonstrations held by the men" in the 19th century. Sunday was chosen so that as many working women as possible could attend. It is claimed that it attracted a crowd of over 300,000. At the time, it was the largest protest to ever have taken place in Britain. Speakers included Marie Naylor, Emmeline Pankhurst, Christabel Pankhurst, Adela Pankhurst, Emmeline Pethick-Lawrence, Mary Gawthorpe, Jennie Baines, Rachel Barrett, Marie Brackenbury, Georgina Brackenbury, Annie Kenney, Nellie Martel, Gladice Keevil, Flora Drummond and Edith New.
In 1909 Naylor worked alongside Annie Kenney, Clara Codd, Lilian Dove-Wilcox, Vera Holme and Elsie Howey in the West of England campaign. During this period she became a frequent visitor to Eagle House near Batheaston, the home of fellow WSPU member, Mary Blathwayt. Her father, Colonel Linley Blathwayt was sympathetic to the WSPU cause and on 9th April 1910 he planted a tree, a Abies Subalpina, in her honour in his suffragette arboretum in a field adjacent to the house.
In November 1911 she broke a pane of glass in a Home Office window. During her trial she claimed that it was the only act of wilful damage that she had ever committed.