He was born to Cristopher Johnson in County Meath, Ireland in 1715. He originally planned a mercantile career, but in 1738 he emigrated to America to manage the land granted to his uncle (Admiral Sir Peter Warren). He settled on the Mohawk River about 25 miles west of Schenectady, New York. He learned from and lived with the Mohawk Indians who adopted him, and later made him a sachem (civil chief). He also began to purchase land from the Indians in his own right.
William was married in 1739 to a German immigrant, Catherine Wisenberg. She originally came as to America as a indentured servant, but he purchased her contract, released and married her and the couple had three children before her death. Their son John Johnson, inherited his father's title and estates. When Catherine died he married Caroline Peters who was the niece of Chief Hendrick and gave him three more children before her death five years later. When Caroline died he married another Mohawk, Molly Brant, sister of the chief Joseph Brant, and she bore him another eight children.
He founded Johnstown, New York on his estate and brought Irish immigrants to New York.
In May of 1750 the king appointed him a permanent member of the Governor's Council for the Colony of New York. He attended the Albany Congress in 1754 which reached an accord that kept the Iroquois on the British side during the next several wars.
French and Indian War
General Braddock as commander-in-chief, commissioned him a Major General and tasked him to lead militia forces against Crown Point. In September, his expedition defeated Baron Dieskau. In recognition of this victory he was awarded £5,000 and made a Baronet in November. In 1758 he was part of General Abercrombie's failed attempt to take Fort Ticonderoga.
He led an Indian and militia force as part of General Prideaux's siege of Fort Niagara in the summer of 1759. When Prideaux was killed, he took command of the force and captured the Fort. He also accompanied General Amherst at the capture of Montreal in 1760. After the war, King George rewarded him with the grant of an additional tract 100,000 acres north of the Mohawk River.
He died from a stroke at his home in Johnstown on July 11, 1774. His manor house is now a New York Historic Site and is open to the public.