On his Virginian estates, Fairfax developed a profitable operation based on the forced labour of several hundred black slaves.

A steadfast Loyalist during the American Revolution, he was largely protected from the loss of his property due to Fairfax's friendship with George Washington.

The property included a large portion of the Shenandoah and South Branch Potomac valleys, and consisted of approximately 5,282,000 acres (21,380 km2) of land.

[3] In the fall of 1732, Fairfax read Carter's obituary in the London monthly The Gentleman's Magazine and was astonished to discover the vast personal wealth Carter had accumulated, which included £10,000 worth of cash, at a time when the governor of Virginia was paid an annual salary of £200.

In 1738, Fairfax established approximately thirty farms in the Patterson Creek Manor, a 9,000-acre (36 km2) piece of land granted to him by the Crown.

[citation needed] The northwestern boundary of the Northern Neck Proprietary, which had been contested by the Privy Council of Great Britain, was marked in 1746 by the Fairfax Stone at the headwaters of the North Branch Potomac River.

Returning to North America in 1747, Fairfax first settled at Belvoir, a slave plantation which had been completed by William six years earlier.

Since, were it not for the Revolutionary War, his immense domain should also have passed to Robert Fairfax, the latter was awarded £13,758 in 1792, by Act of Parliament for the relief of American Loyalists.

[9] The Swan Pond Manor Historic District encompasses land Lord Fairfax set aside in 1747 for his personal use.