The Mothers' Union has within the Church of England long been the butt of jokes and not a little hostility, because of its moral conservatism and, in the eyes of parish clergy, as an autonomous and threatening women's organisation with high ecclesiastical connections. It was founded in 1876 in a rural vicarage drawing room by Mary Sumner, who had grown up in a devout and prosperous Evangelical household and had married into an Anglican ecclesiastical dynasty: her husband's father, at the time of their marriage, was bishop of Winchester and his uncle was archbishop of Canterbury. Mary's father-in-law was one of a group of reforming bishops of the mid-century, anxious to revitalise the institutional and spiritual life of the established Church. It was therefore entirely comprehensible that a serious young clergy wife should envisage an association of mothers bound by a life of prayer and commitment to life-long marriage and motherhood as holy vocations. What it would become – one of the largest global voluntary networks of women dedicated to addressing issues in the lives of women and children – by the end of the twentieth century, would have been beyond her dreams. This new account of the Mothers' Union charts this process, and the acute stresses and divisions which at times threatened its mission and very existence. Here is no hagiography but a professional historian's shrewd analysis based on the rich but barely used archive of the Mothers' Union now housed in Lambeth Palace Library, London, supplemented by English diocesan material and records from three case studies of the Mothers' Union as a local organisation, Australia, South Africa and Ghana. (Mary Sumner had her personal papers burned after her death, possibly to hide the fact that her son had an illegitimate daughter.) The Mothers' Union became the largest Christian women's organisation in the British Isles, its membership peaking just before the 1939–45 war at well over half a million. By the start of the twenty-first century this had dropped to around 100,000 members. However, outside the old heartland the Union expanded rapidly, and by 2008 had a global membership of 3.6 million women in seventy-eight countries. This resilience and expansion reflects the belated ability of the Union to be flexible about its goals and the qualifications for membership. After facing a profound challenge from a secularising society in many countries and a parting of the ways with the moral position adopted by the Church of England on the touchstone issue of divorce, the Mothers' Union's central organisation recognised reality and in 1974 dropped its total opposition to divorce and allowed membership by all baptised women, not discriminating against the divorced or the single. Moreover it granted autonomy to its world-wide branches, enabling them to grow and change as best suited local situations. It thus transformed itself into a Christian women's association dedicated to global social service. This history illuminates many important historical themes which concern both secular and ecclesiastical historians. Of course it is a significant contribution to the history of the institutional and spiritual life of the Church of England and of global Anglicanism in decades of rapid change, and suggests ways which people of different stances have found in order to stay together and recognise that shared faith and goals are more important than issues which divide them. Historians of women's experiences and of gender more broadly will find exceptional material in the records of this organisation, while for historians of empire and the post-imperial world the Mothers' Union is a largely untapped gold mine. It was first an organisation of settler women in the emerging dominions who were eager to support each other and maintain their British identity, but eventually chafed at the restraints imposed on them by the London headquarters. Latterly it has become one of the most important organisations for the empowerment of women in less developed countries who find in it a source of social respectability and status, and a sisterhood through which to tackle major social issues. The case studies presented here show the need for further investigation of the Mothers' Union outside Britain if its historical and contemporary significance is to be understood.