Working in deprived districts of London, notably Deptford, and Bradford, she agitated for reforms to improve the health of young children, wrote several books on nursery education and pioneered a play-centred approach that has only latterly found wide acceptance.
When she was four an epidemic of Scarlet fever killed her father and sister and left Margaret deaf (she recovered her hearing at the age of fourteen).
[citation needed] In 1902 the sisters joined the recently formed Labour Party, working with James Keir Hardie and George Lansbury.
Through McMillan's work and joint campaigning with Katharine Glasier, the House of Commons passed the 1906 Provision of School Meals Act.
In The Child and the State published in 1911, McMillan argued that schools should be offering a broad and humane education instead of preparing children for unskilled, monotonous jobs.
McMillan was injured while protesting the way Women's Social and Political Union members were treated in prison through the Cat and Mouse Act.
[citation needed] In 1914 the sisters founded the Open-Air Nursery School & Training Centre in Deptford for children from eighteen months to seven years and for adult trainees.
McMillan invited personalities like Bernard Shaw and Walter de la Mare to speak to audiences in Deptford.
[6] An award-winning park named after her[7] stands on the site of what was once one of the most deprived streets in Deptford, as shown on the poverty map published by Charles Booth.