The King of Mann (Manx: Ree Vannin) was the title taken between 1237 and 1504 by the various rulers, both sovereign and suzerain, over the Kingdom of Mann – the Isle of Man which is located in the Irish Sea, at the centre of the British Isles.
Since the emergence of Somerled and his descendants in the 12th century, the Manx kings began to lose territory and power in the Hebrides. Before the reigns of the three sons of Olaf the Black, the Manx kings styled themselves "King of the Isles". By the time of the reigns of Olaf's sons, the kings had begun to style themselves "King of Mann and the Isles".
Between 1265 and 1333, Mann was ruled directly by the kings of Scots (1265–1290, 1293–1296, 1313–1317, 1328–1333) or the kings of England (1290–1293, 1296–1313, 1317–1328).
|“||Totum jus et clamium quod habemus, habuimus vel aliquo modo habere poterimus, in Insula de Man cum suis pertinentiis quibuscumque; ita quod nec Nos, nec haeredes nostri, seu quivis alius nostro nomine, aliquid juris vel clamii in Insula praedicta de caetero exigere poterimus vel vindicare.||”|
|“||The whole right and claim that We have, We have had, or in any manner shall We be able to have in the future, in the island of Man with all its privileges, so that neither We, nor Our heirs, or any other in Our name, will not demand or avenge in the future the aforesaid right or claim to the island.||”|
|— Renunciation of claim to Mann by Edward III, 1333|
William le Scrope was executed for treason for his support of Richard II in his struggle with Henry Bolingbroke, who defeated Richard and became Henry IV. Le Scrope's possessions, including the Isle of Man, passed to the Crown.
As Henry's predecessor, Edward III, had recognised Mann as an independent kingdom, 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'.
|“||We have given and granted … to the said Earl of Northumberland the Island, Castle, Peel and Lordship of Man, and all the islands and lordships appertaining to the said Isle of Man, which belonged to Sir William le Scrope deceased, whom in his life We lately conquered, and so have decreed him conquered, and which by reason of that conquest, as having been conquered, We seized into Our hands; which decree and conquest as touching the person of the said William and all his lands and tenements, goods and chattels, as well within as without Our Kingdom, in Our Parliament by the assent of the Lords temporal … at the petition of the Commons of Our said Kingdom, are confirmed …||”|
|— Letters-patent of 19 October 1399|
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. In addition, but separate from the power of governance over the Island, John Stanley was also granted the patronage of the Diocese of Sodor and Man.
A second letters-patent were issued and re-granted to Sir John Stanley on 6 April 1406, the difference being that the grant was inheritable and had a different feudal fee, the service of which comprised rendering homage and a tribute of two falcons to all future Kings of England on their coronations.
Edward Stanley, 3rd Earl of Derby, the son of Thomas Stanley, 2nd Earl of Derby, did not take the style 'King', and he and his successors were generally known instead as Lord of Mann. However, the Latin style Rex Manniae et Insularum (King of Mann and the Isles) continued to be occasionally used in official documents until at least the 17th century.