In 1940, Clarke sought permission from Cardinal William Henry O'Connell, Archbishop of Boston, to establish an educational center near Harvard University.
[6] On January 17, 1949, Clarke, Father Feeney, and Fakhri Maluf founding the Slaves of the Immaculate Heart of Mary, a Traditionalist Catholic religious community.
[5] In one of her books, Clarke wrote that "in absolute literalness, we must admit that it is possible for a human being to lose his soul without being guilty of any sin committed by himself.
[4][10] In May 2020, Patricia Walsh Chadwick, a former member of the Slaves of the Immaculate Heart of Mary, accused Clarke of physically abusing her and other children that grew up in the religious community.
[11][6] Clarke wrote multiple books on Catholic history, theology, and spirituality including Our Glorious Popes, Charlemagne and the Finding of the Body of St. Anne, The Pontificate of Pope Saint Leo the Great, The Life of Saint Gregory the Great, Gate of Heaven, The Failure of Interfaith, Love Is The Spirit Of Truth, and The Loyolas and the Cabots.