He became known through trade with the Indians by learning their language and customs as well as treating them fairly.

He held the rank of Major General in the British Militia and distinguished himself in the border warfare with the French. 1748, he was put in command of all the New York colonial forces. In 1755, he was appointed "Sole Superintendent of the Affairs of the Six United Nations, their Allies and Dependents". He held this position until his death, July 11, 1774.

Sir William became interested in Joseph Brant, the Mohawk Indian Chief, and later took Brant's sister, Molly, as his common-law wife. He was subsequently knighted for his service in the French and Indian War and given 100,000 acres of land in the Mohawk Valley.