Guy Johnson (c.1740 – 5 March 1788) 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 his successor in 1774. The following year, 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. Accused of falsifying reports, he went to London to defend himself after the war, and died there in 1788.
Guy was the son of either John or Warren Johnson of Smithstown, Dunshaughlin, Co. Meath, each younger brothers of Sir William Johnson. The Johnsons were descendants of the O'Neill dynasty of Ireland.
In 1756, he sailed from Ireland and joined his uncle William in the Mohawk Valley of the Province of New York. He assisted his uncle, who was British Superintendent of Indian Affairs for the northern colonies. He was agent to the Iroquois, with whom the British had a strong trading diplomatic relationship.
On 1763, Guy Johnson married William's daughter Mary (Polly), one of his children by his first consort, Catherine Weisenberg. His uncle (now also father-in-law) gave them a square mile of land on the Mohawk River, located in what is now Amsterdam. In 1773, their first home was destroyed by a lightning strike.
They replaced it in 1774 with a large limestone house in the Georgian style, which they called Guy Park. Soon after, they were forced to leave because of rising tensions in the area prior to the American Revolution. With other Loyalists, they went to Canada to settle near Fort Niagara. On the way, Polly Johnson died at Oswego.