William Keith was the son of Robert Keith, Master of Marischal and Lady Elizabeth Douglas, daughter of John Douglas, 2nd Earl of Morton. He was one of the earls who accompanied James V to France for his marriage to Madeleine of Valois, daughter of King Francis I of France.
The wedding took place on 1 January 1537 at Notre Dame Cathedral in Paris and was followed by days of Jousting at the Louvre. He fought at the Battle of Pinkie in 1547, in which the Scots were defeated by the English forces led by Edward Seymour, 1st Duke of Somerset in a bloody battle that was part of the War of the Rough Wooing. Keith was said to have been in favour of the proposed marriage between the infant Mary, Queen of Scots, and Prince Edward of England, son of King Henry VIII. He held the Office of Extraordinary Lord of Session in 1541, 1561, and lastly in 1573.
Later, the Earl Marischal retired with his great fortune to a secluded life at Dunnottar Castle becoming known locally as "William of the Tower." The Earl died on 7 October 1581.
In 1561, he was a member of Queen Mary"s Privy Council.
