The Legion of Mary was founded in Dublin in 1921 by Frank Duff and a group of fifteen women. Focusing on evangelical and social work, and set against the rise of Catholic Action in the wider Church, it developed from a single group in a Dublin parish to become a major international organisation and one of the largest lay movements in Catholicism. Surprisingly, the Legion rarely features in Irish historiography and no critical study has been produced. Drawing on the archives of the Legion, as well as state and Church archives in Ireland and Britain, this dissertation aims to rectify this omission by assessing the origins, motivations and development of the Legion between its foundation and the Second Vatican Council. The Legion’s most prominent activities, which differed from place to place according to local need, included an apostolate to prostitutes, hostels for the homeless, home visitation, evangelical and catechetical work, missionary service, ecumenical outreach and work with Irish emigrants in Britain. A defining aspect of the Legion was the centrality it placed on the role of the laity in the mission of the Church, predating the thinking of the Vatican Council. Initially founded for women, the Legion heralded the place of women in the Church. Attracting members from all classes, it demanded strict equality among them. Extending beyond Ireland to Britain in 1928, and then around the world from 1931, it became a significant, effective and influential global Catholic movement. In its work, as it sought humane solutions for social problems, it challenged an inherent lack of compassion in Irish society. The influence of Duff was substantial, he became the essence of the Legion. His innovative thinking, organisational skills, and openness to new ideas and experimentation in pastoral, social and evangelical work, rooted the Legion in deep, even radical theological, spiritual and historical traditions, and moved it beyond being a mere manifestation of Catholic Action to something unique, universal and effective.