William Maxwell was a Presbyterian of Scottish descent born in County Tyrone, Ireland in about 1733. By 1747 his family had come to North America, and settled in Warren County, New Jersey. When the French and Indian War broke out in 1754 Maxwell enlisted in the provincial militia, and served in the disastrous expedition of General Edward Braddock. He served as an ensign in Col. John Johnson's N.J. Regiment and then a lieutenant in Col. Peter Schuyler's regiment, the Jersey Blues, and was likely on the 1758 campaign that culminated in the debacle of the Battle of Carillon. At the end of the war he remained in military service, serving on the western frontier.