Thomson Mason served in the late 1750s and again from 1766 to 1772, participating in the legislative world in which opposition to parliamentary taxation developed. In 1774 he represented Loudoun at the first Virginia convention, although he opposed its nonimportation resolution and withdrew from public activity for a time, citing poor health. Voters elected him to the House of Delegates for Loudoun in 1777–1778, and he subsequently represented Elizabeth City County and Stafford. In 1778 the legislature appointed him to the General Court as Virginia reorganized its judiciary after independence. His judicial tenure was short, and he resumed legislative and legal work. His estates depended on enslaved labor, with dozens of enslaved people recorded among his holdings near the end of his life. 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. Perhaps as a result of her influence, Elizabeth City County voters elected Mason as one of their representatives in the Virginia House of Delegates in early 1779. Mason died on 26 February 1785 at Chopawamsic at the age of 51. Part of Raspberry Plain Plantation survives today as part of the Catoctin Rural Historic District, although the house that Thomson Mason built and that his descendants altered, burned and was torn down in the early 20th century and replaced in 1910 by the current edifice in the Colonial Revival style.