The Legion of Mary is an association of Catholic laity who serve the Church on a voluntary basis.It was formed on 7 September 1921.  It was founded in Dublin, Ireland, as a Roman Catholic Marian Movement by layman Frank Duff. Today between active and auxiliary (praying) members there are over 3 million members worldwide making it the largest apostolic organization of lay people in the Catholic Church

To be in the Legion of Mary, one must be a practising Catholic. Active members serve God under the banner of Mary by practising the Spiritual Works of Mercy. The main apostolate of the Legion consists in activities directed towards all men and women, young and old, rich and poor as well as people from the margins of society and towards non-Catholics. The members of the Legion are primarily engaged in the performance of the Spiritual Works of Mercy rather than works of material aid.