Sir Dafydd's family, always loyal to the English Crown, were opponents of the Welsh rebellion formed by Owain Glyndwr in 1400. While fighting for the English, during the rebellion, Dafydd came to know the young Prince Henry who would assume the English throne in 1413.

Dafydd Gam fought for Henry V at Agincourt where he saved the King's life during the struggle between Henry and France's Duke of Alencon. During the combat Henry's brother, the Duke of Gloucester, was wounded and it was Dafydd's sword that ended Alencon's life. However, saving Henry's life cost Sir Dafydd his own.