By the mid-1740s, McGillivray was well established as a trader in the Upper Creek nation in what is now central Alabama. He established a fur trading post and plantation at Little Tallassee (also spelled "Talisi" in some documents) near today's Wetumpka, Alabama, possibly on the site of the former Fort Toulouse. He prospered and invested his trading and plantation profits in businesses on the Atlantic coasts of Georgia, eventually settling in Savannah, Georgia, as a man of considerable wealth. In a will drafted in 1767, long before his death, he planned the disposition of a plantation on Hutchinson Island, Georgia, a plantation known as "Vale Royal" upriver from Savannah, and cash bequests totaling more than £2,500, implying that he was in possession of that amount of currency, as well as numerous bequests of slaves and other valuable chattel.