With the start of the American Revolution in 1776, young Thomas Thornhill joined the Virginia Line of the Continental Army. He was around 16 or 17 years old, and first served as a private in Lee's Legion.
Lee's Legion was under the command of Henry "Lighthorse Harry" Lee, father of Confederate General Robert E. Lee. An ebook on Lee's Legion is available at https://archive.org/stream/LeesLegionRemembered/.
At some point after 1777, Thomas transitioned into Captain John Roberts' company. Thomas appears on the roll for Roberts' company in 1779.
Thomas Thornhill filed a pension application from his service in Revolutionary War. I will have a whole post on his pension application, which provides interesting information on the Thornhill family in Virginia.