The Quebec Act, passed by Parliament in 1774, made the former New France into a single British colony called the Province of Quebec. At the time, the Ontario region was largely an unsettled wilderness; settlement did not begin until after the outbreak of the American Revolution.

Thousands of United Empire Loyalists, American colonists who chose to remain under British rule, migrated to central Canada during and after the Revolutionary War. They were to have an important influence on the development of Ontario and were to bring about its separation from Quebec. The Loyalists were accompanied by a number of Iroquois Indians who were allies of the British during the war.

The large influx of English-speaking settlers into the Province of Quebec, the eastern portion of which was predominantly French, led to its division into two provinces. By the Constitutional Act of 1791, Quebec was divided at the Ottawa River—all of the western territory becoming Upper Canada (mainly English) and the eastern territory becoming Lower Canada (chiefly French). In 1792 the first assembly of Upper Canada met at the capital, Newark (now Niagara-on-the-Lake). In 1793 Lieutenant Governor John Graves Simcoe selected York (now Toronto) to be the permanent capital. Simcoe also encouraged more American immigration. At the outbreak of the War of 1812, more than three-fifths of the people were immigrants from the United States.

During the War of 1812, as in the Revolutionary War, attempts were made by the United States to conquer Canada. Important battles took place at Queenston Heights (1812), Stoney Creek (1813), and Lundy's Lane (1814).

The period from the end of the war to mid-century was one of rapid expansion. New towns were founded, including Ottawa and Windsor. Roads and canals were built. In 1829 the Welland Canal made navigation possible between Lakes Erie and Ontario by providing a waterway that bypassed Niagara Falls. The Rideau Canal, which linked the Ottawa River with Lake Ontario, was completed in 1832. Between 1815 and 1840, population increased from about 90,000 to more than 400,000. The vast majority of the immigrants who came during this period were British; most became farmers.

For a generation after the War of 1812, there was political strife as settlers sought democratic rights. Although Upper Canada and Lower Canada had representative institutions, they did not have what was termed responsible government—the real power lay with autocratic executive and legislative councils who were appointed by the Crown and who were not responsible to (that is, dependent on the support of) the popularly elected legislative assemblies.

Early attempts at reform were made in the assemblies—but met with no success. By 1837 the dissent had become open rebellion. William Lyon Mackenzie led an uprising in Upper Canada, and a second revolt, led by Louis Joseph Papineau, erupted in Lower Canada. Both rebellions were put down, but Britain recognized that reform was needed.

In 1839 Lord Durham, who had been sent from Great Britain to assess the political situation, recommended the union of Upper and Lower Canada, largely to absorb the French and to facilitate economic progress. The result was the Act of Union of 1840 (effective 1841). He also advised granting responsible government; this recommendation was carried out in 1849. Under the Act of Union, Upper Canada and Lower Canada became the Province of Canada with Upper Canada (Ontario) as an administrative district called Canada West.