[2] Membership is highest in South Korea, Philippines, Brazil, Argentina and the Democratic Republic of Congo, which each have between 250,000 and 500,000 members.

[4] The Legion of Mary was founded by Frank Duff on 7 September 1921 at Myra House, Francis Street, Dublin.

[5] His idea was to help Catholic lay people fulfil their baptismal promises to be able to live their dedication to the Church in an organized structure, which would be supported by fraternity and prayer.

Most notably, the Legion began helping Dublin’s prostitutes in the Monto, the largest red light district in Europe at the time, to run away from their "kip-keepers" and start new lives.

Duff also set up a similar refuge for unwed mothers, but defied the usual consensus in Ireland at the time by teaching the women how to raise their children instead of giving the babies away for adoption.

On 27 March 2014, the Secretary of the Pontifical Council for the Laity, Bishop Josef Clemens, delivered the decree in which the Legion is recognized by the Holy See as International Association of the Faithful.

[6] In 2021, the Legion celebrated the centennial of its founding amidst the COVID-19 pandemic, by hosting virtual talks, devotions, and masses.

The group also resolved to increase its membership and engagement among young people and to pray for the beatification of Servants of God Frank Duff, Alphonsus Lambe, and Venerable Edel Quin.

[7] The next level is the Comitium, which is in charge of several Curiae, usually over an area like a medium city or part of a province.

[8] Each level of the Legion has the same set of officers: the President, the Vice-President, the Secretary, the Treasurer, and the Spiritual Director.

Active members regularly attend weekly sessions of their Praesidium and pray daily the Catena Legionis, which consists essentially of the Magnificat and some shorter prayers.

It has a statue of the Virgin Mary (represented as standing on a globe, her arms extended, crushing the serpent with her foot), which is placed upon a white tablecloth with "Legio Mariae" written on it.

Sessions begin with introductory prayers to the Holy Spirit and to Mary, including five decades of the Rosary.

The vexillum is made out of metal and onyx, and shows the Holy Spirit in the form of a dove, as well as the Miraculous Medal.

[9] The book promotes a "total dedication" to Christ through devotion to the Blessed Virgin Mary, which later influenced popes such as John Paul II, who mentions it in an apostolic letter, Rosarium Virginis Mariae.

The idea of a Catholic lay apostolate organization where ordinary laypeople in all situations of life would work towards personal sanctification and for the conversion of the world was the first of its kind.

[citation needed] After the Second Vatican Council (1962–65) promoted such ideas in its conciliar documents, this approach gained wider acceptance in the Catholic Church.