Sir William Marshall 3rd Earl of Pembroke was born about 1145 in Pembroke, Pembrokeshire, Wales. He died May 14, 1219 in Caversham Manor, England. He was buried in May 1219 in Round Chapel Of Knight's Temple, London, Middlesex, England. He married Isabel De Clare who was born 1172 in Pembroke, Pembrokeshire, Wales. He was Grand Marshal under King John I. His counselor at the Magna Carta Signing in 1215. Chief negotiator for King John at Runnymede. The Regent for King Henry III until his death in 1219. He was Instrumental in having John crowned King (although he despised him). Knighted 1164. After King John died, he was Regent for King Henry III. Defeated Prince Louis of France Army. Negotiated a peace treaty and remained the Regent until his death. Shortly before he died he joined the Knights Templar.