Henry Percy, 1st Earl of Northumberland, 4th Baron Percy, titular King of Mann, KG, Lord Marshal (10 November 1341 – 20 February 1408) was the son of Henry de Percy, 3rd Baron Percy, and a descendent of Henry III of England. His mother was Mary of Lancaster, daughter of Henry, 3rd Earl of Lancaster, son of Edmund, Earl of Leicester and Lancaster, who was the son of Henry III.
As King Edward III of England had recognised Mann as an independent kingdom, his successor Henry IV did not directly claim the Manx throne, but instead proclaimed that he had acquired the island by right of conquest, which in international legal theory at that time erased any existing constitutional arrangements. He then on 19 October 1399 granted the Island, as a fiefdom under the English Crown, to Henry Percy, 1st Earl of Northumberland together with wide-ranging powers of government and associated regalities, together with the style of "Lord of Man", in a position of feudality and thus without sovereignty. Despite this, Percy styled himself as "King of Mann".
Following Percy's treasonous rebellion, Henry IV granted the suzerainty of the Isle of Man, on similar terms but only for the term of his life, to Sir John Stanley in 1405.