When did Cornwall become part of England?

Answer 1

Cornwall became part of England after the Norman conquest in 1066 and was included in King William's Domesday Book.

However some activists argue that Cornwall is governed by the Duchy of Cornwall, rather than the English Crown. In fact, Cornwall is recognised as English land and therefore under the ownership of the English Crown. The Duchy of Cornwall ceased to exist as soon as William of Normandy took control of Mainland Britain.

It was during the Tudor period that some disgruntled Cornwall residents began to argue that Cornwall was not part of England. A false belief which is still spread today.

So, your answer is that Cornwall officially became part of England in 1066 when William the Conqueror became the king of England, all of mainland Britain apart from Wales and Scotland came under the rightful control of the English Crown.