John O. Thornhill, the son of Jesse Thornhill, was born in Virginia around 1834. Enlisting in the 2nd Virginia Cavalry of the Confederate Army on May 13, 1861 as a private during the American Civil War, Thornhill was elected first lieutenant of his company on May 8, 1863 and wounded at the battle of Aldie on June 17 of that year. He had returned to duty by March 1864 and was wounded again at Todd's Tavern on March 7, 1864. Thornhill seems to have remained absent on medical leave for the remainder of the war. His post-war occupation is listed as farmer. John O. Thornhill died in Campbell County, Virginia on December 17, 1907.

Organized from counties in west central Virginia, the 2nd Virginia Cavalry of the Confederate Army (originally designated the 30th Virginia Cavalry) was composed primarily of units that had previously served in the commonwealth's militia. (Lynchburg's "Wise Troop," which would become the 2nd Virginia's Company B, had been a militia unit since 1859.) The regiment initially served in northern Virginia and participated in the first battle of Bull Run. The 2nd later joined Jackson's 1862 Valley Campaign, then was ordered to the Virginia peninsula, where it fought in the Seven Days Battles. Remaining in Virginia, the regiment saw heavy fighting and participated in several major battles, including Chancellorsville, Spotsylvania, and Cold Harbor. The 2nd spent several months protecting rail lines near Petersburg, then joined Early's 1864 Valley Campaign. Returning east in early 1865, the regiment participated in the Appomattox Campaign. The 2nd Virginia Cavalry was disbanded at Lynchburg on April 11, 1865.