Sir Alexander Mackenzie (or MacKenzie, Scottish Gaelic: Alasdair MacCoinnich; 1764 – 12 March 1820) was a Scottish explorer known for accomplishing the first east to west crossing of North America north of Mexico, which preceded the more famous Lewis and Clark Expedition by 12 years. His overland crossing of what is now Canada reached the Pacific Ocean in 1793. The Mackenzie River, the longest river system in Canada and the second longest in North America, is named after him.

1789 Mackenzie River expedition to the Arctic Ocean

On behalf of the North West Company, Mackenzie traveled to Lake Athabasca where, in 1788, he was one of the founders of Fort Chipewyan. He had been sent to replace Peter Pond, a partner in the North West Company. From Pond, he learned that the First Nations people understood that the local rivers flowed to the northwest. Acting on this information, he set out by canoe on the river known to the local Dene First Nations people as the Dehcho, (Mackenzie River) on 10 July 1789, following it to its mouth in the hope of finding the Northwest Passage to the Pacific Ocean. As he ended up reaching the Arctic Ocean on 14 July,^[7][8] it is conjectured that he named the river "Disappointment River" as it did not lead to Cook Inlet in Alaska as he had expected.^[3] The river was later renamed the Mackenzie River in his honor.

1792–93 Peace River expedition to the Pacific Ocean