Prior to European colonization, the Thousand Islands region was home to, or visited by, members of the Iroquois Confederacy and Ojibwa people. Their name for the islands was Manitouana or the "Garden of the Great Spirit".
The region was a part of the War of 1812 between the British Empire and the United States. Many sites from the war can be found, such as Fort Wellington in Prescott, Ontario and the garrison on Chimney Island, Mallorytown, Ontario. Museums about the war can be found on both the Canadian and American side of the river.
Historical postcard views of Alexandria Bay (left) and a steamboat touring Lost Channel, c. 1900
In the late 19th and early 20th centuries, many distinguished visitors made the region widely known as a summer resort. During the half century (1874–1912) of the resort's greatest prominence, most wealthy vacationers came from New York City, joined by prominent families from Chicago, Cleveland, Pittsburgh and other cities of the United States and Canada. Several grand hotels provided luxurious accommodations while steamboats offered extensive tours among the islands. Wealthy and middle-class summer residents built summer homes, and the region retains a historically important collection of vacation homes from this time.
Among the lavish homes built during this time were several masonry "castles", some of which remain as international landmarks. The region's first castle, Castle Rest, was built in 1888; it was destroyed in the mid-20th century. The most famous extant examples are "The Towers" on Dark Island, now called Singer Castle, and the previously long-neglected Boldt Castle on Heart Island, which had been left unfinished for over 75 years upon the untimely death of George Boldt's wife. It has since been completed over the recent decades in accord with Boldt's original plans (Wikepedia).