The funny thing about Ontario is it started as a part of Quebec. Until the division of Upper and Lower Canada in 1791, this was all the one unified colony that Britain took from France in the conquest of 1760. Settlers started moving in 1783 first from central New York in the first direct Loyalist wave, then over the rest of the decade from the eastern US seacoast as the losers of the Revolution filtered their way around up from New York, Nova Scotia and then down the St. Lawrence. An oblique reference in a land document for a Lieut. Mackay from 1794 gives an interesting hint as to the development of brewing here in Kingston, the commercial center of this new colony:
On May 27, 1794, a petition had been presented in Council on his behalf for "a Piece of Land about the usual Size of a Town Lot, situated on the West side of a Lot lately laid out for the Kingston Brewery, to be bounded on the North by the said Brewery on the East by a small run of Water, on the South by the Common, & on the West by the top Bank..."
The passage is from The Parish Register of Kingston Upper Canada 1785-1811, an online resource that also confirms that by 1797 it was managed by one John Darnley. That is it above shown on an 1824 map. It is also shown on a map from 1865 and later additions are still there - as the 2003 photo at this post shows.. There was earlier brewing in the colony but likely tied to taverns like Finkle's in nearby Bath. Steve Gates, our comment leaver and author of the excellent book The Breweries of Kingston & The St. Lawrence Valley pegs the building of the brewery in 1793 by merchant John Forsyth but it is John's brother Joseph who is more of the man about Kingston in the 1790s. But, as a garrison town and a depot supplying deep into the continent from the main colonial centre of Montreal, it is entirely likely that their Kingston business affairs overlapped repeatedly.
Creation of the brewery reflected some level of certainty after years of difficulty in ensuring the grain crops could supply the expanding colonial population. A 1796 letter noted in Preston's Kingston Before The War of 1812 even speaks of the continuing infestation of Hessian Fly affecting the area. Building a brewery spoke to an expectation of peace.