In 1820, when William McGillivray realised that the collapse of the North West Company (NWC) was imminent unless an agreement could be made with their rivals, the Hudson's Bay Company (HBC), Simon took a leading role. Together with his friend Edward Ellice, they devised a plan to merge the two giant fur companies. During the discussions that followed, Colin Robertson remarked: "I like Simon much better than his friend the Member of Parliament (Ellice); there is a sort of highland pride and frankness about the little fellow that I don't dislike". The merger was completed by 1821, and having broken the news to the partners in Canada, Robertson again commented, "Simon McGillivray has carried everything without even the semblance of opposition. The first day he opened the business, the second the Deed and Release was signed, and the third all was peace and harmony".
