On December 4, 1915, it was announced that a new Archdiocese of Winnipeg was to be formed out of part of the Archdiocese of St. Boniface. However, the new archbishop, Most Reverend Alfred Arthur Sinnott, D.D., was not installed until December 24, 1916.
One reason for this delay was that the Vatican had to deal with St. Boniface's concerns regarding the boundary between the two archdioceses. The boundary was finally set to follow a line along the west shore of Lake Winnipeg and the Red River southward into the city of Winnipeg. It then turned west along the Assiniboine River to a point between Portage la Prairie and Brandon before turning south again to the American border. This boundary was very similar to that requested in the 1906 petition.
Although the creation of the Archdiocese of Winnipeg left St. Boniface as the Metropolitan See for Manitoba, the Archdiocese of Winnipeg was not made subject to St. Boniface. Instead, Winnipeg became the only archdiocese in Canada immediately subject to the Holy See.