[1] Their father died in a ferry accident when his sons were boys, but their mother supervised operations of the family's plantations (farmed using enslaved labor) as well as acquired land in what became Prince William, Fairfax and Loudoun Counties for her younger sons (since primogeniture meant their eldest brother George would receive the lands his father inherited).
Thomson Mason like his brothers received a private education suitable to his class, then traveled to Williamsburg, for studies at the College of William & Mary.
In 1758 he also became Barnes' partner in some of the voyages in which Virginia or Maryland tobacco was shipped to Europe and slaves imported from Africa on the return journey.
[4] In 1760, Mason had purchased a plantation he would call Raspberry Plain in Loudoun County, Virginia, which he operated (like Chopowamsic) using enslaved labor.
[6] Thus, during the First Virginia Revolutionary Convention in 1774, Mason represented Loudoun County, together with fellow plantation owners Francis Peyton and Josias Clapham.
[7] However, Thomson Mason became the only delegate to oppose a nonimportation resolution, and decided to retire from public life, citing ill health.
Three years later, Loudoun County voters elected (and re-elected) Mason and Clapham as their (part-time) representatives in the Virginia House of Delegates during the 1777 and 1778 sessions.