The county was named in 1792 to honour Henry Dundas, who was Lord Advocate for Scotland and Colonial Secretary at the time. Matilda and Williamsburgh 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.
James Pliny Whitney, Ontario's sixth premier, is buried here in the cemetery of Holy Trinity Anglican Church in Riverside Heights, just east of Morrisburg and north of County Road 2 (formerly Highway 2). Whitney was born in Williamsburg in 1843, represented Dundas County in the Legislature from 1888 to 1914 and served as Premier from 1905 to 1914.
Morrisburg and Iroquois were partially flooded by the creation of the St. Lawrence Seaway in 1958. Unlike the Lost Villages of Cornwall and Osnabruck Townships, however, the two towns were simply relocated to higher ground in the same area. There was an international design competition in 1954 to design the new Iroquois townsite. Canadian-British architect Wells Coates was among those who submitted redevelopment concepts.
An artificial lake, Lake Saint Lawrence, now extends from a hydroelectric dam at Cornwall to the control structure at Iroquois, and replaces the formerly narrow and turbulent section of river that was impassable to large vessels. It replaces, in part, the Long Sault rapids.
In 1976, stuntman Ken Carter attempted to jump a one mile portion of the Saint Lawrence River by taking a one million dollar Lincoln Continental rocket car off an eight-storey ramp. This was billed as The Super Jump. The ramp and its runway were located in a field just west of Hanes Road, South of County road 2. The ramp has since been demolished, but the concrete runway still exists as of 2012.
Charles A. Barkley, who was elected mayor of the municipality in the 2006 municipal elections, died unexpectedly on June 17, 2009. He was a municipal politician since 1981 when he joined the Township of Matilda council. He was succeeded by deputy mayor Robert Gillard.
Most family history material will be found under the names of the original townships.