Northumberland County was first settled by indigenous peoples, including the Cayuga peoples, who migrated there from New York in 1779.
Shortly after this, European settlers, primarily Loyalists who relocated from the United States after the American Revolution, began to settle in the area. The British government rewarded the loyalty of these people by providing them with plots of land for nominal sums, on the condition that they would work the land to eventually turn it into good farmland.
After the War of 1812 more British settlers were wanted, and port towns like Cobourg and Port Hope became important centres for commercial activity. However, from the 1880s to the 1950s, economic growth was slow in some areas of Northumberland County. This is why many old buildings have been able to be preserved as heritage sites, especially in Port Hope, which is often considered to have the best-preserved 19th century streetscape in Ontario.