John Porteous, a Scotsman by birth, arrived in America from Perth, Scotland, in 1762. In 1764, he was hired as a clerk by John Duncan, a Schenectady fur trader. The next year, he began working for James Sterling in Detroit and at Fort Michilimackinac in present-day Mackinaw City. His duties included selling goods, buying furs, and extending credit, and collecting debts. In this role, Porteous also made occasional travels to Canada and New York. He was also associated with the merchants James Phyn and Alexander Ellice, and before the start of the Revolution, established himself as an important trader in Montreal.
During the American Revolution, Porteous worked as a merchant in New York City, and helped to supply the local garrison with British goods. He was also part-owner of the British privateer Vengeance . At the time of the British evacuation, he returned briefly to Scotland, before settling in Nova Scotia, and subsequently, Little Falls, New York. There, he continued to work as a merchant and also operated a flour mill. He died in March 1799.