Thomas Stanley, 1st Earl of Derby, KG (1435 – 29 July 1504) was titular King of Mann, an English nobleman and stepfather to King Henry VII of England. He was the eldest son of Thomas Stanley, 1st Baron Stanley and Joan Goushill, and through his mother a lineal descendant of King Edward I by Elizabeth Plantagenet, Countess of Hereford and the FitzAlan family of Arundel.
A landed magnate of immense power, particularly across the northwest of England where his authority went almost unchallenged, even by the Crown, Stanley managed to remain in favour with successive kings throughout the Wars of the Roses until his death in 1504. His estates included what is now Tatton Park in Cheshire, Lathom House in Lancashire, and Derby House in the City of London, now the site of the College of Arms.
Although the king for the early part of his career, Henry VI, was head of the House of Lancaster, Stanley’s marriage to Eleanor, daughter of Richard Neville, 5th Earl of Salisbury (a descendant of Edward III) and sister of Richard Neville, 16th Earl of Warwick (‘Warwick the Kingmaker’) in the late 1450s constituted a powerful alliance with the House of York. This did him no harm, however, even after Warwick was toppled from power, and in 1472, with the House of York now occupying the English throne, he married his second wife Lady Margaret Beaufort, Countess of Richmond and Derby , whose son, Henry Tudor, was the leading Lancastrian claimant. He was the last to use the style ‘King of Mann’, his successors opting for the safer ‘Lord of Mann’. Stanley was “a man of considerable acumen, and probably the most successful power-broker of his age”.
More information on the Wikipedia page .