His Eminence Frank Cardinal Leo was born in the city of Montreal, Quebec, Canada on 30 June 1971 to Italian immigrant parents †Francesco Leo and †Rosa Valente. He is the second of two brothers, Anthony being the elder. Cardinal Leo attended Eugenio Pacelli Elementary School, John F. Kennedy High School and CEGEP Vanier. He entered the Grand Séminaire de Montréal in 1990 and was ordained a priest for service to the Archdiocese of Montreal on 14 December 1996. He served in different parish assignments in Montreal: assistant pastor at Our Lady of Consolata, parochial administrator of Saint-Joseph-de-RDP, and pastor of Saint-Raymond-de Peñafort until 2006 when he accepted the invitation to enroll in the Pontifical Ecclesiastical Academy in Rome and subsequently entered the Diplomatic Service of the Holy See where he served in Apostolic Nunciatures across the globe from 2006 until 2012. On 9 January 2012, Pope Benedict XVI appointed him Chaplain of His Holiness, bestowing the title of Monsignor.

Upon his return to Canada, he joined the formation team of the Grand Séminaire de Montréal, teaching theology and philosophy while providing spiritual direction, formation and accompaniment to the candidates for the priesthood. In 2015 he was appointed General Secretary of the Canadian Conference of Catholic Bishops (CCCB), a mandate which came to an end in 2021. On 1 February 2022, he was named Vicar General and Moderator of the Curia of the Archdiocese of Montreal.

Cardinal Leo was appointed by the Holy Father as Titular Bishop of Tamada and Auxiliary Bishop of Montreal on 16 July 2022. He was ordained to the Episcopacy on 12 September 2022 in Mary Queen of the World Cathedral in Montreal. His episcopal motto, taken from the Gospel according to Saint John (John 2:5) is: “Quodcumque Dixerit Facite” (in English: “Do Whatever He Tells You”).

Named as Metropolitan Archbishop of Toronto on 11 February 2023 by Pope Francis, he was installed as Archbishop on 25 March 2023. On 29 September 2023 the pallium was conferred onto him at St. Michael's Cathedral Basilica by the Apostolic Nuncio to Canada, the Most Reverend Ivan Jurkovič. On 6 October 2024 Cardinal Leo was appointed by Pope Francis to the College of Cardinals with the Consistory that took place on 7 December 2024.

In addition to his extensive graduate studies in Canon Law (Pontifical Lateran University) and Diplomacy and International Law, Cardinal Leo holds a Doctorate in Systematic Theology (University of Dayton ⁄ IMRI) with a specialization in Mariology, a Licentiate in Philosophy (Pontifical Lateran University), a Diploma in Classical Studies (Université de Montréal) and a Graduate Certificate in Spiritual Direction (Aquinas Institute of Theology). He worked as a Judge for the Canadian Appeal Tribunal and has taught theology, spirituality and philosophy in Montreal, Canada; Canberra, Australia; Dayton, United States of America; and Ottawa, Canada, among other places.

Cardinal Leo speaks English, French, Italian and Spanish. He is president and founding member of the Canadian Mariological Society and is also a member of the Priestly Fraternity of St. Domenic (Third Order Dominican).

His Eminence is Chancellor of the Pontifical Institute of Mediaeval Studies of Toronto; Chancellor and Chair of the Board of Governors of St. Augustine's Seminary of Toronto; Chair of the Board of Directors of the Redemptoris Mater Missionary Seminary of Toronto; and Chancellor of the University of St. Michael's College in the University of Toronto.

Furthermore, Cardinal Leo is Apostolic Chancellor of Catholic Missions in Canada; Ex officio member of the Canadian Council of Catholic Bishops (CCCB) Permanent Council as well as member of the National Episcopal Commission for Doctrine; Member of the Education Commission of the Assembly of Catholic Bishops of Ontario (ACBO); Member of the Board of Directors of Catholic Near East Welfare Association (CNEWA) Canada; Grand Prior of the Equestrian Order of the Holy Sepulchre of Jerusalem, Canada Toronto Lieutenancy; Bailiff Grand Cross of Honour and Devotion for The Sovereign Military Order of Malta - Canadian Association, and Member of the Knights of Columbus. He received the King Charles III Coronation Medal in August 2025.

For the Holy See, Cardinal Leo was appointed as Member of the Dicastery for Legislative Texts in January 2025, a Member of the Dicastery for Interreligious Dialogue in July 2025, and a Member of the Dicastery for Clergy in August 2025.