William Fairfax, born in 1691 in England, a cousin of Lord Thomas Fairfax, and doubly related by marriage to George Washington, was second only to Lord Fairfax as the major early landowner in present West Virginia. William purchased Lord Fairfax's manors of Shannondale, South Branch Creek, and Patterson Creek, now in the Eastern Panhandle, to support his Belvoir plantation on the lower Potomac. At Belvoir, William Fairfax entertained the Washingtons, including his son-in-law, Lawrence Washington, George Washington's older brother and guardian.

William had served from India to New England, where he was collector of customs at Boston until 1732, when Lord Fairfax chose him as agent for his Virginia estate (more than five million acres, including most of the present Eastern Panhandle). Experienced, active, and honest, William was invaluable to Lord Fairfax in the colony, regarding relations with the Virginia government, land speculators, and settlers. At his urging the Fairfax estate was finally officially surveyed and its limits ascertained, as witnessed by the Fairfax Line of 1746.

In 1748, William launched 16-year-old George Washington on his career as surveyor, sending him to accompany his son, George William Fairfax, to Patterson Creek, to set boundaries for German speaking settlers there. In 1754, William was commanding the Alexandria volunteers against the French and Indians in the Shenandoah Valley when he was incapacitated by illness.

William Fairfax died September 3, 1757, and was buried at Belvoir. His son, Bryan, became the Eighth Lord Fairfax.