[1][2] He shared the same name as his uncle Thomson Mason, his father's younger brother who became a prominent lawyer, politician and judge until his death in 1785, and also owned and operated plantations using enslaved labor, mostly in Loudoun County.
[1][2] The couple had eight children:[1][2] Through deeds of gift in 1781 and 1786, Mason's father passed to him ownership of four tracts totaling 676 acres (2.74 km2), together with slaves.
[4] While his brother George was in Europe trying to recover his health, Thomson operated his plantations, thus gaining experience using enslaved labor.
[4][6] Thomson Mason represented Fairfax County in both houses of the Virginia General Assembly.
Voters in Fairfax and neighboring Prince William County elected him to the Virginia Senate in 1800 and re-elected him to another four year term,[7] and he ended his legislative career in the Virginia House of Delegates with a single term in 1808.