In 1792 he was transferred to Fort Fork (Peace River, Alberta) on the request of Alexander Mackenzie.
In 1808 MacKay resigned from the NWC and returned to the East to retire wealthy at age 38 in Montreal.
[1] In 1810 Mackay, along with several other retired NWC personnel, such as Donald McKenzie and Duncan McDougall, signed a preliminary agreement with the American businessman John Jacob Astor.
Astor intended to establish a new fur trading company to operate in the Columbia River region.
All of these people joined MacKay in his 1811 sea voyage to the mouth of the Columbia River on the Tonquin.
Then in June 1811 he sailed as supercargo on the Tonquin, and led an expedition that attempted to acquire furs from natives along the coast to the north.