Sir Alexander Mackenzie (Scottish Gaelic: Alasdair MacCoinnich; 1764 - March 12, 1820) was a Scottish-Canadian explorer. Mackenzie was born in Stornoway on the isle of Lewis in the Outer Hebrides, Scotland. In 1774 his family moved to New York, and then to Montreal in 1776 during the American Revolution. In 1779 he obtained a job with the North West Company, on whose behalf he traveled to Lake Athabasca and founded Fort Chipewyan in 1788. He was 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 and discovered the Mackenzie River on July 10, 1789, following it to its mouth in the hope of finding the Northwest Passage to the Pacific Ocean. Although he ended up reaching the Arctic Ocean, he named the river "Disappointment River" as it did not lead to Cook Inlet in Alaska as he had expected. The river was later renamed the Mackenzie River in his honour.

In 1791 he returned to Great Britain to study the new advances in the measurement of longitude. Upon his return in 1792 he set out once again to find a route to the Pacific. Accompanied by native guides, French voyageurs and a dog called "Our Dog", Mackenzie left Fort Fork following the route of the Peace River. He found the upper reaches of the Fraser River, but was warned by the local natives that the lower portion of the river was unnavigable and populated by belligerent tribes.[1] He was instead directed to follow an established trading route by ascending the West Road River, crossing over the Coast Mountains, and descending the Bella Coola River to the sea. He followed this advice and reached the Pacific coast on July 20, 1793 at Bella Coola, British Columbia, on North Bentinck Arm, an inlet of the Pacific Ocean. Thus, he completed the first recorded transcontinental crossing of North America, north of Mexico. The previous most northern crossing had been by Cabeza de Vaca of the Narváez expedition, beginning on the Gulf Coast of Texas and concluding on the Gulf of California in Mexico. [2] In the process MacKenzie crossed the Continental Divide. He had wanted to continue westward out of a desire to encounter the open Ocean, but was turned back by the hostility of the Heiltsuk nation. The expedition of George Vancouver had visited Bella Coola 48 days before. At his westernmost point on Dean Channel, (on July 22, 1793), hemmed in by Heiltsuk war canoes, he inscribed "Alexander Mackenzie from Canada, by land, the twenty-second of July, one thousand seven hundred and ninety-three!" on a rock using a reddish paint made of vermilion and bear grease, and turned around to return to "Canada".[3]:418 The rock, near the water's edge in Dean Channel, still bears similar words, which were permanently inscribed later by surveyors. The site is now Sir Alexander Mackenzie Provincial Park.

Inscription at the end of the Alexander Mackenzie's Canada crossing located at 52°22'43?N 127°28'14?W? / ?52.37861°N 127.47056°W? / 52.37861; -127.47056[4]He was knighted for his efforts in 1802, and served in the Legislature of Lower Canada from 1804 to 1808. In 1812, he married and returned to Scotland. Mackenzie died in 1820 of Bright's disease. He is buried in Avoch, on the Black Isle, Ross and Cromarty.