[6] In 1827 Fauquier county voters elected Thomas Marshall as one of their two delegates in the Virginia General Assembly (alongside Alexander D. Kelly) and he began his state legislative service on December 3, 1827.

On January 6, 1830 John Macrae resigned as one of Fauquier County's delegates to the Virginia Constitutional Convention of 1829-1830 and Thomas Marshall replaced him for the final sessions.

Then Fauquier voters elected and re-elected Marshall to the Virginia House of Delegates, so he began his longer stint of legislative service on December 6, 1830, which ended with his death in 1835.

His youngest son and namesake, Lt. Col. Thomas C. Marshall Jr., would buy Oak Hill from his elder brother John (who died in 1854), then volunteered to fight with the Confederate States Army.

Initially commissioned as a captain with the 7th Virginia Cavalry, the junior Thomas Marshall initially trained and fought in a local militia company led by Turner Ashby, had become an aide to General Stonewall Jackson at the First Battle of Manassas (which Col. Ashby missed), and would earn the rank of Col. as well as have six horses shot from under him and be wounded twice before dying in a skirmish in November 1864.