[clarification needed] The diocese was created on December 17, 1841, out of the Roman Catholic Archdiocese of Kingston, and it covered the western half of Upper Canada.

During the 1840s, the major challenge was the huge unexpected influx of very poor immigrants, mostly Irish escaping the Great Famine.

In response, the Church built a network of charitable institutions such as hospitals, schools, boarding homes, and orphanages, to meet the needs and keep people inside the faith.

McGowan argues that between 1890 and 1920, the city's Catholics experienced major social, ideological, and economic changes that allowed them to integrate into Toronto society and shake off their second-class status.

Starting as unskilled labourers, they used high levels of education to move up and were well represented among the lower middle class.

On March 19, 2019, the Ontario Court of Appeal allowed a former altar boy to proceed with suing the Archdiocese of Toronto on grounds related to sex abuse.

As the regional municipalities expand, the northern section of the Archdiocese, Simcoe County, is also experiencing notable suburban growth.