Martin's uncle Thomas Fairfax, 6th Lord Fairfax of Cameron (known as Lord Fairfax) owned the Northern Neck Proprietary land grant, which he had inherited from Martin's great-grandfather Thomas Colepeper, 2nd Baron Colepeper in 1719. The proprietary constituted up to of Virginia's Northern Neck and a vast area spanning west to the North Branch Potomac River headwaters. The Northern Neck Proprietary had been awarded by the exiled Charles II of England to seven of his supporters in 1649, and again in 1688 by official patent. One of these seven supporters, Lord Colepeper, acquired the right to the entire proprietary in 1681, and his grandson, Lord Fairfax, inherited the land grant upon the death of his mother, Katherine Colepeper. Lord Fairfax dispatched his first cousin William Fairfax (1691–1757) to replace Robert Carter I as the steward and land agent for the Northern Neck Proprietary, a position in which Fairfax served until his death in 1757. In 1750, Martin's uncle Lord Fairfax established himself at his hunting plantation Greenway Court estate near present-day White Post in Clarke County with the intention of administering the proprietary himself. Lord Fairfax had previously reserved this land as his private residence in 1747, then known as "the Quarter". With his cousin William Fairfax acting as land agent, Lord Fairfax sought additional assistance in managing the proprietary and scrutinized which of his family members from England would be up to the task. Lord Fairfax first considered his brother Robert, then his brother-in-law Denny Martin, and in 1751, he finally decided upon enlisting the assistance of his twenty-year-old nephew and Denny Martin's son, Thomas Bryan Martin.