Mason sought to expand his land and wealth. He greatly expanded the boundaries of Gunston Hall estate, so that it occupied all of Dogue's Neck, which became known as Mason's Neck. One project that Mason was involved in for most of his adult life was the Ohio Company, in which he invested in 1749 and became treasurer in 1752—an office he held forty years until his death in 1792. The Ohio Company had secured a royal grant for to be surveyed near the forks of the Ohio River (today the site of Pittsburgh, Pennsylvania). War, revolution, and competing claims from Pennsylvania eventually defeated the Ohio Company's plans. Although the company failed, Mason acquired considerable western lands independently. His defense against the Pennsylvania claims, "Selections from the Virginia Charters" (1772), originally intended to promote the Ohio Company's claims, was widely applauded as a defense of the rights of Americans against royal decrees. Involvement with the Ohio Company also brought Mason into contact with many prominent Virginians, including his Fairfax County neighbor, George Washington.