There is a former cathedral, St. Mary’s Pro-Cathedral, also in Hamilton and a minor basilica, Our Lady Immaculate, in Guelph, Ontario.

It was established on 29 February 1856 by Pope Pius IX as the Diocese of Hamilton, on territory split off from the Archdiocese of Toronto, which became its Metropolitan.

Special signs, marks and posters were commissioned for many of the diocese's churches, schools and buildings.

The Diocese of Hamilton had begun as a Catholic Mission in Upper Canada (Ontario).

There are 6 deaneries (Brant, Bruce-Grey, Halton, Hamilton, Waterloo and Wellington) which have 126 parishes in their geographical grouping.