Frank Duff (1889–1980) was born Francis Michael Duff on June 7, 1889, in Dublin, Ireland, the eldest of seven children. Raised in a devout Catholic household and educated at Blackrock College, he entered the Irish Civil Service at 18. While serving in the Society of St. Vincent de Paul at age 24, he became deeply aware of poverty in Dublin’s tenements—experiences that shaped his lifelong devotion to the poor and marginalized.

Inspired by his reading of St. Louis de Montfort’s True Devotion and impelled by his growing Marian spirituality, Duff, alongside Fr. Michael Toher and a group of women, founded the Legion of Mary on September 7, 1921, in Dublin. This lay apostolic society—modeled after a military legion—was dedicated to spiritual growth through prayer, active charity, and service to the Church, especially under Mary’s guidance. Under Duff’s leadership, the Legion swiftly expanded beyond Ireland, establishing hostels for former prostitutes and the homeless, and by the 1930s had spread internationally.

A renowned figure in lay evangelization, Duff spent over a decade guiding the Legion full-time after retiring from the civil service in 1934. He was a lay observer at Vatican II in 1965—receiving a standing ovation—and remained active in promoting devotion to the Sacred Heart and the rosary until his death on November 7, 1980. Today, with millions of active and auxiliary members worldwide, the Legion of Mary remains his enduring legacy.