Born to an Irish feudal family, William's father John was Marshal to the king. At age 13, William was sent to France to serve as squire to his father's cousin, the Chamberlain of Normandy. As he matured and learned the skills of knighthood, William proved to be an exceptional student and progressed to become a talented knight much feared in tournament combat. William Marshal on his deathbed recalled having bested 500 knights in tournaments.

In 1170 with the endorsement of Eleanor of Aquitaine, wife of Henry II, Marshal was made the military tutor of Henry's oldest son, the young Henry. A great rapport developed between the seasoned knight and his young charge.

After Prince Henry's death in 1183, Marshal remained at court and became military and political advisor to King Henry. When a civil war developed between Henry and his sons, Marshal defeated the rebellious youths and spared the life of Richard, who would succeed Henry and be called "Lionheart." When Richard became king, William was his loyal and faithful servant, and Richard rewarded him with estates and wealth.

After Richard died fighting in France in 1199, William Marshal went on to serve his unvirtuous brother John. Through the loss of Plantagenet lands in France, a revolt by the barons, warfare with the French and repeated travails, Marshal remained loyal to John. When John died in 1216, he left Marshal as Regent of England for his son Henry.

William Marshal died in 1219, a much honored figure.