There was a Thomas Henry Johnston in Botetourt Co, VA (not my line but he kept coming up during my research since he had the same name, same rank in CSA, at the same time as my gg gf in MS) (also ironically, he married a cousin of my T. H. Johnston) anyway,

Thomas Henry Johnston was born 1836 and married Sarah Watson Minor Holladay. Thomas was son of Edward Johnston (b. 1807) and Emmeline Sophia Bowyer (b. 1809).

Edward and his brother, John Nash Johnston, were sons of Andrew Johnston and a Nash wife. Andrew is son of Peter Johnston (b. 1710 in Scotland) and Martha Butler. Andrew had brothers: Charles (m. Lettia Pickett and Elizabeth Steptoe) and Peter (m. Mary Wood, neice of Patrick Henry the famous patriot and statesman)