 He retired from the Civil Service in 1934 to devote all of his time to the Legion of Mary.Inspired by St. Louis de Montfort's "True Devotion to Mary", he founded the Legion of Mary.Along with a group of Catholic women as well as Michael Toher, a priest of the Dublin Archdiocese, Duff formed the first branch of what was to become the first praesidium of the Legion of Mary on 7 September 1921.