Mackenzie Sir Alexander (1764–1820), a Scottish-Canadian fur trader and explorer. He discovered the Mackenzie River in 1789 and followed it to its mouth at the Arctic Ocean. In 1793 he completed the first crossing of North America north of Mexico, by traveling westward across Canada to the Pacific Ocean.

Mackenzie was born in Scotland. He was taken to New York City in 1774, became a clerk for a Montreal fur company in 1779, and later became a trader and partner. Mackenzie was made a partner of the North West Company in 1787. In 1789 he led an expedition to search for a northwest passage to the Pacific. The party left from Fort Chipewyan on Lake Athabasca and traveled by canoe on the Slave River, Great Slave Lake, and Mackenzie River, to the Arctic Ocean. The 3,000-mile (4,800-km) round trip was made in only 102 days.

In 1792 Mackenzie crossed from Montreal to Fort Chipewyan to begin a new search for the Pacific Ocean. Mackenzie's party left the fort in 1793, paddling west on the Peace River through totally uncharted territory. When the river divided into two forks, they followed its southern branch, the Parsnip River. From here the route became more tortuous. They followed the Fraser, Blackwater, Bella Coola, and Dean rivers, but often they had to travel on foot. They reached the Pacific Ocean at Dean Channel (about 300 miles [480 km] northwest of what is now Vancouver, British Columbia) The round trip took 108 days.

Returning east, Mackenzie acquired a fortune by managing fur-trade companies. He was knighted in 1802 and was a member of the Lower Canada (Quebec) assembly, 1804–08. He returned to Scotland in 1808.

Mackenzie's account of his travels was published in 1801.