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Born around 1760 to a proud but impoverished Scottish family, Mackay immigrated to North America as a teenager. He soon traveled west to seek his fortune in the fur trade. For the next decade, Mackay traversed thousands of miles of wilderness. At one point he traveled up the Northern Saskatchewan River as far west as present-day Edmonton; on another journey he spent 17 days walking south from a point near Winnipeg across the snow-covered plains to the great villages of the Mandan Indians (just north of today's Bismarck, N.D.).

Unlike many foot soldiers in the fur trade, Mackay kept journals and made maps, information that would be invaluable on the expedition for which he became famous. But in the short term, the ambitious young man felt only frustration at his subordinate role. Eager to make his own mark, in the 1780s Mackay traveled to Britain to seek funding for an independent fur-trading scheme. The specifics of his plans have been lost -- as were, eventually, the investments of his partners.

Undeterred, Mackay moved to New York, where he finagled a meeting with Don Diego Maria de Gardoqui, Spanish minister to the United States. At the meeting, Mackay pitched the idea of a new Spanish expedition up the Missouri to the Pacific Ocean. To prove he was the man for the job, Mackay presented Don Diego with his own map of the seldom-charted land of the Mandan.

"This person," Don Diego wrote to a foreign service colleague, "although a young man, says he has traveled for nearly five years from the English establishment of Hudson Bay towards the west among various Indian nations, crossed the Mississippi and the Missouri and arrived at the cordillera of mountains which divide the waters, some to this ocean and others to the Pacific."

Clearly such a man would be valuable to King Carlos IV, who was eager to achieve a more rigorous enforcement of Spain's monopoly on regional commerce. Traders from France, the United States and especially Britain had been openly defying Spain's authority for years, and Spanish merchants were furious. Spain was also keen to find the "Northwest Passage," the waterway that 18th century geographers were convinced traversed the continent. Laying claim to this fabled route would be the biggest prize of all.

Mackay returned to the St. Louis area in 1791 and two years later became a naturalized Spanish citizen. When the Spanish finally got around to mounting a new voyage up the Missouri in 1795, Mackay's connections and experience paid off: "Because of his knowledge of the northern fur trade, he was the best person in St. Louis to lead the expedition," Wood says.