In addition to Iroquois and Morrisburg, the township comprises the communities of Archer, Beckstead, Boucks Hill, Brinston, Dixons Corners, Dunbar, Dundela, Elma, Froatburn, Glen Becker, Glen Stewart, Grantley, Haddo, Hainsville, Hoasic, Hulbert, Irena, Mariatown, Muttonville, New Ross, Nudell Bush, Oak Valley, Orchardside, Pleasant Valley, Riverside Heights, Rowena, The Sixth, Stampville, Straders Hill, Toyes Hill, Williamsburg and Winchester Springs.

The county was named in 1792 to honour Henry Dundas, who was Lord Advocate for Scotland and Colonial Secretary at the time. Matilda and Williamsburg were two of Upper Canada's original eight Royal Townships. The northern portions of Matilda and Williamsburg townships were separated in 1798 to form the new townships of Mountain and Winchester within Dundas County.

The McIntosh apple was invented, or discovered, in South Dundas near Williamsburg. John McIntosh's parents emigrated from Inverness, Scotland to the Mohawk Valley in New York and John moved to Upper Canada in 1796. In 1811 he acquired a farm in Dundela, and while clearing the land of second growth discovered several apple seedlings. He transplanted these and one bore the superior fruit which became famous as the McIntosh Red apple. John's son Allan established a nursery and promoted this new species extensively. It was widely acclaimed in Ontario and the northern United States and was introduced into British Columbia about 1910. Its popularity in North America and propagation in many lands attest the initiative and industry of John McIntosh and his descendants.