Thomas de Felton was with the expedition commanded by Edward III which invaded France in 1346, and he took part in the Battle of Crécy, the capture of Calais and the other important events of that campaign. He became an important figure in the service of Edward the Black Prince. When Prince Edward went to take possession of Gascony in 1355, Felton went with him, and followed him to the Battle of Poitiers. His marriage to Joan de Walkfare and the birth of his children followed in the later 1350s. He was one of the commissioners who signed the Treaty of Brétigny (1360) and took oath to see it executed. He is named among the principal witnesses to the marriage of Prince Edward to Joan, Countess of Kent in 1361, as "miles" (i.e., Knight), and appears elsewhere as "chivaler", through most of his career.