It is still inhabited by the descendants of 18th and early 19th-century Scottish Highland pioneer settlers from Lochaber, was historically a Gàidhealtachd community, and Canadian Gaelic language revival efforts are currently taking place there.

Glengarry was founded in 1784 by Gaelic-speaking United Empire Loyalists, mainly from Clan Donald, whose defeat in the American Revolution had caused them to become refugees from the Mohawk Valley in upstate New York, North Carolina, and, despite the fact that most Scottish Gaels in that Colony chose to be Patriots, from Georgia.

[1] His Majesty's Government, as represented by the Governor General of British North America, Guy Carleton, 1st Baron Dorchester, hoped the new migrants would help settle and develop the area, which first became known as Upper Canada and later as Ontario.

The Crown accordingly issued land grants and helped with supplies during the first winter, in lieu of financial compensation for their confiscated properties in United States.

Following an 1814 visit to the settlement, Dr. D. MacPherson wrote, "You might travel over the whole of the County and by far the greater part of Stormont, without hearing anything spoken except the good Gaelic.

For example, poet Anna NicGillìosa (1759-1847) emigrated from Morar to Upper Canada in 1786 and eventually settled in South Glengarry, and a Gaelic song-poem in praise of her new home there survives.

"[9] By the same decade, Glengarry County had also become a major center for outward migration, especially to the United States, "In proportion to size of territory and population, the district has sent more lumbermen to Michigan forests, more settlers to Minnesota prairies, more hands to assist and direct the construction of railways, than any other on the American continent... effective reference may be made to the settlement in Dakota, where a new Glengarry is springing up.