In 1767, he was removed from active service at Fort Sinclair, which he had built in 1764 under orders from Colonel John Bradstreet.
He never recovered financially and spent his remaining years on his estate drawing his half pay from the military and also from his time as lieutenant-governor of Michilimackinac.
As a private citizen, Sinclair acquired a large tract of land, which he called the Pinery, on the west bank of the St. Clair River in eastern Michigan.
In 1780, Jean Baptiste Point du Sable, a prisoner at Fort Michilimackinac was the caretaker of the property.
Du Sable and his wife, Kitiwaha, a Potawatomi Indian, oversaw the Pinery for four years, living in a cabin at the mouth of the Pine River in what is now the city of St. Clair, Michigan.