Prior to 1904, Catholics in Northern Ontario were under the jurisdiction of the Diocese of Peterborough. Faced with an ever-growing Catholic population, Bishop Richard O'Connor saw the need to divide the territory and create another diocese. The idea was presented to Pope Pius X and the Diocese of Sault Ste. Marie was officially founded in 1904.  

The Diocese of Sault Ste. Marie was formally established on September 16th, 1904 and the City of Sault Ste. Marie was chosen for the new cathedral because it was geographically in the middle of the diocese.  The Most Reverend David Joseph Scollard was ordained to the Episcopacy on February 24th, 1905 and became the first bishop of diocese. 

At its founding, the territory of the diocese extended west from Callander, Ontario for 1280 kilometres, almost reaching the Manitoba border.  During the episcopacy of the second Bishop, Bishop Ralph Hubert Dignan (1935-1958), the western end of the diocese was separated on August 26th, 1952 to form the Diocese of Port Arthur, now known as the Diocese of Thunder Bay.  

The Diocese of Sault Ste. Marie now comprises the Districts of Sudbury and Algoma; and includes portions of Nipissing District north and west of Lake Nipissing, and also includes Manitoulin Island. The diocese has three major population centres Sault Ste. Marie, Sudbury and North Bay.