In 1773, McGill joined with Isaac Todd (who would remain a close lifelong friend) in a trading venture beyond Grand Portage, which was renewed under the name Todd & McGill in 1776. This partnership was important in the formation of what would become the North West Company. Todd & McGill prospered, as one of the main firms supplied by the London commission merchant, John Strettell. The partnership did not participate in the North West Company after 1783, but it continued in the so-called "Southwest trade" in the Mississippi valley until Michilimackinac was handed over to the Americans in 1796. It was also involved in other enterprises, but few business papers have survived, making a detailed account difficult.