In creating the Apostolic Vicariate of Temiscaming, on 22 September 1908, from the Diocese of Pembroke, the Church takes root in Northern Ontario.

January 7, 1916, the Vicariate becomes the Diocese of Haileybury.

April 18, 198, the Apostolic Vicariate of Northern Ontario is created from dismemberment of the Diocese of Haileybury.

December 3, 1938, the Vicariate of Northern Ontario is dismembered for the  creation of the Diocese of Hearst and the creation of the Apostolic Vicariate of James Bay. This Vicariate becomes the Diocese of Moosonee in 1967.

December 3, 2018, the territories of the diocese of Hearst and of Moosonee are merged and the two dioceses amalgamated to create the new diocese of Hearst-Moosonee.

Bishops of Hearst

1919 – 1939 • Bishop Joseph Hallé, Prefect and Apostolic Vicar of Northern Ontario (+October 7, 1939)

1939 – 1940 • Bishop Joseph Charbonneau, First Bishop (+November 19, 1959)

1940 – 1945 • Bishop Albini Leblanc (+May 17, 1957)

1946 – 1952 • Bishop Georges-Léon Landry (+December 29, 1977)

1952 – 1964 • Bishop Louis Lévesque (+March 12, 1998)

1964 – 1973 • Bishop Jacques Landriault (+November 6, 2017)

1973 – 1993 • Bishop Roger Despatie (+May 14, 1994)

1994 – 1995 • Bishop Pierre Fisette, p.m.é. (+December 21, 1995)

1996 – 2006 • Bishop André Vallée, p.m.é. (+February 28, 2015)

2006 – 2016 • Bishop Vincent Cadieux, o.m.i.

2016 – 2018 • Bishop Robert Bourgon

Bishops of Moosonee

1938 – 1964 • Bishop Henri Belleau, o.m.i., First Apostolic Vicar (+January 5, 1976)

1964 – 1967 • Bishop Jules Leguerrier, o.m.i.

1967 – 1992 • Bishop Jules Leguerrier, o.m.i., First Bishop, (+June 8, 1995)

1992 – 2016 • Bishop Vincent Cadieux, o.m.i.

2016 – 2018 • Bishop Robert Bourgon, Apostolic Administrator

Bishops of Hearst-Moosonee

2018 – 2020 • Bishop Robert Bourgon

2020 – present • Archbishop Terrence Prendergast, S.J., Apostolic Administrator