When Anderson left to take control of the Tredegar Iron Works in Richmond, Thomas was promoted to brigadier general to command the brigade.

He was a graduate of Oxford College of Emory University and served in the Mexican–American War from May 1847 until August 1848 as a second lieutenant in an independent company of Georgia mounted men.

The regiment was attached to Brigadier General Joseph R. Anderson's brigade, which was initially stationed in Georgia but during the Peninsula Campaign was sent to the Richmond area to reinforce Joe Johnston's army.

Thomas then became permanent commander of the brigade, and on November 1 he was promoted to brigadier general, participating in all of the major battles and campaigns fought by the Army of Northern Virginia.

After being in poor health for some time, Thomas died on March 8, 1898, in South McAlester, Indian Territory, and is buried in Kiowa, Oklahoma.