What Is Prince William’s Last Name
Although Royals don't tend to use a last name, in the instance of a marriage, for example, Prince William's last name is Mountbatten-Windsor. This is the surname of all descendants of Prince Philip and Queen Elizabeth II (excluding females who marry).
The name Mountbatten-Windsor derives from the Royal surname Windsor, decided in 1917 by George V. Before this, the Royal surname was technically Saxe-Coburg-Gotha, but due to anti-German sentiment after WWI, the decision was made to change the royal surname to Windsor.
In 1960, Queen Elizabeth II and Prince Philip decided to create a surname for their direct descendants, incorporating the surnames "Windsor" and Prince Philip's previous surname, "Mountbatten."
Therefore, Mountbatten-Windsor is the surname of all male (and unmarried female) descendants of Prince Philip and Queen Elizabeth II; this includes Prince Charles, Prince William and Prince Harry.