Until 1791, all the lands north of the St. Lawrence River and the Great Lakes were simply known as "Quebec." In 1791, the Constitutional Act divided and renamed this vast territory. The Ontario area was called Upper Canada, and the remainder of old Quebec was called Lower Canada. The first capital of Upper Canada was Newark (now Niagara-on-the-Lake), near the Canada-U.S. border. The capital was moved in 1793 to York (now Toronto) which was less vulnerable to raids from the new republic to the south. Upper Canada's first Lieutenant Governor was General John Graves Simcoe.

At the time, there were about 15,000 people in Upper Canada. Over the next 20 years various groups arrived, including German settlers from New York State, Mennonites who settled in the Grand River Valley, and the Catholic Highlanders who settled in Glengarry County. By the time war broke out in 1812, the population had swelled to about 90,000.

In 1812 the United States, angered by British harassment on the Atlantic Ocean, trade problems, and wanting to add Britain’s North American colonies to the United States, declared war on Great Britain and invaded Upper Canada. The Americans were beaten back in 1812. However, the Legislative Buildings, in Toronto, were burned by the Americans in 1813.

The British retaliated in 1814 by invading Washington and burning the Capitol and the president’s house. (The building was dubbed the White House because it was painted white to hide the scorch marks.) The war ended in a stalemate and the Treaty of Ghent was signed that year.