Joachim joined the Women Social & Political Union (WSPU) in 1907. Committed to militant action she was arrested in February 1908, after taking part in a demonstration on the House of Commons and was sentenced to six weeks' imprisonment.
On her release she published an article in Votes for Women where she argued: "What one finds on joining the WSPU is, that one is brought into contact with a great number of people whose ideals are the same as one's own, and that the isolation and the reproach are things of the past."
On 30th June, 1908, she was arrested again after being part of a WSPU deputation to Parliament. This time she was sentenced to three months' in Holloway Prison. On her release she wrote that "the ranks of militant suffragettes are mostly recruited from the mild vegetarians, and the authorities have allowed us a special vegetarian diet".
In February 1909, Joachim worked alongside Ada Flatman in Aberdeen. Nine months later she was arrested with Helen Archdale and Adela Pankhurst in Dundee after interrupting a meeting being held by the local MP, Winston Churchill. In court Joachim was sentenced to ten days' imprisonment. She then became the first woman in Scotland to go on hunger strike. On her release she was immediately involved in militant active. Although she was arrested several times during the next few months but she was always released without charge.
During this period she became a regular 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 17th June 1910 he planted a tree, a Thujopsis Dolabrata, in her honour in his suffragette arboretum in a field adjacent to the house.
On 4th March, 1912 the WSPU organised another window-breaking demonstration. This time the target was government offices in Whitehall. According to Votes for Women: "From in front, behind, from every side it came - a hammering, crashing, splintering sound unheard in the annals of shopping... At the windows excited crowds collected, shouting, gesticulating. At the centre of each crowd stood a woman, pale, calm and silent." Over 200 suffragettes were arrested and jailed for taking part in the demonstration. This included Maud Joachim, who was sentenced to six months' imprisonment. She was sent to Holloway Prison but was later transferred to Maidstone Prison. The Home Office reported that Joachim "is a a person of some influence with the others and is fomenting trouble in Holloway."
The summer of 1913 saw a further escalation of WSPU violence. In July attempts were made by suffragettes to burn down the houses of two members of the government who opposed women having the vote. These attempts failed but soon afterwards, a house being built for David Lloyd George, the Chancellor of the Exchequer, was badly damaged by suffragettes. This was followed by cricket pavilions, racecourse stands and golf clubhouses being set on fire.
Some leaders of the WSPU such as Emmeline Pethick-Lawrence, disagreed with this arson campaign. When Pethick-Lawrence objected, she was expelled from the organisation. Others like Maud Joachim, Elizabeth Robins, Jane Brailsford, Laura Ainsworth, Eveline Haverfield and Louisa Garrett Anderson showed their disapproval by ceasing to be active in the WSPU and Hertha Ayrton, Lilias Ashworth Hallett , Janie Allan and Elizabeth Garrett Anderson stopped providing much needed funds for the organization.
Sylvia Pankhurst was also very critical of the arson campaign. She was also unhappy that the WSPU had abandoned its earlier commitment to socialism and disagreed with Emmeline and Christabel Pankhurst's attempts to gain middle class support by arguing in favour of a limited franchise. In 1913, Pankhurst, with the help of Joachim, Keir Hardie, Julia Scurr, Mary Phillips, Millie Lansbury, Eveline Haverfield, Lilian Dove-Wilcox, Nellie Cressall and George Lansbury, established the East London Federation of Suffragettes (ELFS). An organisation that combined socialism with a demand for women's suffrage, it worked closely with the Independent Labour Party. The group also began production of a weekly paper for working-class women called The Women's Dreadnought.
According to Elizabeth Crawford, the author of The Suffragette Movement (1999): "In August, when war was declared, she (Maud Joachim) was with Sylvia Pankhurst in Dublin. They returned to London immediately and Maud Joachim became secretary of an unemployment bureau set up by the East London Federation of Suffragettes, appealing for work for the local unemployed, and was also manager of the ELFS toy factory until edged out by a more dominant personality."