When Bishop Alexander Macdonell asked Lieutenant Governor Sir Francis Bond Head for land to build a church, he was granted a lot (Park Lot 32)[1] on the north east corner of the garrison reserve, given in part due to it having served as an emergency cholera cemetery during the epidemics that hit York (Toronto) in 1832 and 1834.

Eventually, structural problems developed with the second building and John Walsh, Bishop of the Diocese of London, Ontario and a former pastor of St. Mary's, engaged architect Joseph Connolly to design a new church.

Della Vagna was born in 1801 in Genoa, and before joining the Franciscans at the age of twenty-four, he had worked in his father's counting house and become fluent in English.

Bishop de Charbonnel had often requested he come to Canada and finally having obtained permission of his superiors, Della Vagna arrived in April 1856.

De Charbonnel immediately put him in charge of St. Mary's, but the friar died of an "Inflammation of the lungs" less than a year later on 17 March 1857.

[3] Connolly composed a three-aiselled basilica plan for the brick Gothic Revival design of St. Mary's Church, with a polygonal apsidal sanctuary, transepts slightly lower than the nave, and a morning chapel to the liturgical north or geographic south.

Beneath the central rose window is a blind arcade, and the main doorway below is topped with a gable with two roundels on either side.

The brick- and copper-clad spire is characterized by corner niches, with gables rising above belfry openings between the angle turrets.

Connolly's Church of Our Lady Immaculate in Guelph, Ontario stands similarly at the head of that city's Macdonell Street.