At the age of five or six William was given to King Stephen as a hostage. Stephen's kindness prevented William's death. William was trained as a squire in the Tancarville family, an important family in Normandy. William's allegiance to Henry II and his sons, the Young King and Richard (I) is complicated, swapping sides from time to time, but he had the respect of them all. William was also loyal to king John, supporting him against his many enemies. When king John died, William was named as the guardian of John's son Henry (III), saying that 'the land will never be held by anyone except with his help'. After the death of king John, William re-issued the Magna Carta in 1216 to re-affirm the power of the monarchy and he removed any reference that might impose restrictions on this. In 1217 the Magna Carta again was amended to clarify the feudal content. William assisted Henry III retain the throne when it was under threat from Louis VIII of France.
When William married Isabel de Clare the heiress of the Earl of Pembroke he took the title as well as ownership of Pembroke Castle. William started major reconstruction work at the castle in stone.
William Marshal died in 1219 leaving the country united under the monarchy. His eldest son, also called William, inherited the title of Earl of Pembroke. William Marshall is buried in the round church of the London Temple and his tomb can still be seen there.