It was named after Henry Dundas, 1st Viscount Melville, who was the British Home Secretary (1791–1794), with responsibility for the colonies.
Dundas was first settled by individuals of European background in 1784, when German Loyalists who had fought with Sir John Johnson in the American Revolutionary War re-settled in Canada.
The settlers were descendants of the Palatine immigrants to America in 1710.
In 1792, Dundas County was formally established by a proclamation of the Lieutenant Governor of Upper Canada, John Graves Simcoe, which established several counties in Upper Canada including Dundas.
The boundaries of Dundas county were defined as follows: That the third of the said counties be hereafter called by the name of the county of Dundas; which county is to be bounded on the east by the westernmost boundary line of the county of Stormont, on the south by the river St. Lawrence, and on the west by the easternmost boundary line of the late township of Edwardsburgh, running north twenty-four degrees west until it intersects the Ottawa or Grand river, thence descending the said river until it meets the northwesternmost boundary of the county of Stormont.