Guy was an Irish-born military officer and diplomat for the Crown during the American War of Independence. He had migrated to the Province of New York as a young man and worked with his uncle, Sir William Johnson, British Superintendent of Indian Affairs of the northern colonies. He was appointed as William's successor in 1774. In 1775 Johnson relocated with Loyalist supporters to Canada as tensions rose in New York before the American Revolutionary War. He directed joint militia and Mohawk military actions in the Mohawk Valley. He traveled to London with Mohawk Chief Joseph Brant early in the war to appeal his case to be able to command the tribes in Canada, as well as the northern colonies but was unsuccessful. Accused of falsifying reports, he went to London to defend himself after the war, and died there in 1788.
His uncle William provided a square mile of land on the Mohawk River, located in what is now Amsterdam, NY. In 1773, the first home was destroyed by a lightning strike. He replaced it in 1774 with a large limestone house in the Georgian style, which was called Guy Park. The home is still standing, but was heavily damaged by flooding after Hurricane Irene in August, 2011.
Guy's parentage is unclear. He was the son of either John or Warren Johnson, younger brothers of Uncle William. He married his first cousin Mary (Polly), one of William's daughters. Thus William became his father-in-law as well as his uncle.
Source Wikipedia article https://en.wikipedia.org/wiki/Guy_Johnson citing numerous original sources.