The first MGH was built at the corner of Craig Street (today St. Antoine) and St. Lawrence Boulevard and only had 24 beds.

Having outgrown this space, it moved to a new 72-bed building on Dorchester Boulevard (now René-Lévesque) between St. Dominique and De Bullion Streets; today this facility is a long-term care centre.

At this time, Montreal had two hospitals: the Grey Nuns' Hopital General and the Hôtel-Dieu de Montréal).

In 1819 enough money had been received to lease a building on Craig Street to accommodate 24 patients and this small hospital opened on May 1, 1819.

Famous Montreal beer-brewer John Molson of "Molson's beer" (founded in 1786 and still sold to this day under the name "Molson Canadian Lager") and his three sons John Jr., Thomas, and William contributed to the hospital financially through the purchasing of the Dorchester Street lot, the construction of the main building itself, and physically - with John Molson being the presenter of the initial petition to the Legislative Assembly of Lower Canada (after being sick and seeking hospital care himself, becoming personally invested) to create a new public hospital in Montreal in 1820, however the petition would be denied only because of a procedural error.