George William Fairfax (1724–1787), a relative of Thomas, Lord Fairfax, proprietor of the Northern Neck of Virginia, was the the son of William (1691-1757) and Sarah Walker Fairfax. He was educated in England, and returned in 1746 to America to join his father in Virginia. George William lived at Belvoir, an estate about four miles from Mount Vernon, and during his years of residence there, he and GW became close friends. George William held several offices, including colonel in the Frederick County militia, 1755-56. In the summer of 1773 he made an extended visit to England in order to look after property that he had inherited there. GW agreed to oversee his business affairs in Virginia during his absence and received his power of attorney on 8 July 1773. GW continued to supervise his American interests until the outbreak of the Revolution. Battaile Muse, GW's land agent, later handled Fairfax's business affairs in Virginia, to which place Fairfax never returned. He remained in England for the remainder of his life.

George William married Sarah (“Sally”) Cary Fairfax (c.1730–1811) in December 1748.