The first of this family mentioned is John de Plessets, an eminent Norman who came to England in the beginning of the reign of Henry III. He became a domestic servant in the court of King Henry III, and, having served in the Welsh wars, was constituted governor of the castle of Devizes, in Wiltshire, and warden of the forest of Chippenham, in the same shire. In the 24th King Henry's reign [1240] he was sheriff of Oxfordshire, and in two years afterwards he had a grant of the wardship and marriage of John Bisset, and likewise of the heirs of Nicholas Malesmaines. Certain it is that he enjoyed in a high degree the favour of his royal master for, upon the death of John Mareschal, who had m. Margery, the sister and heir of Thomas de Newburgh, Earl of Warwick, the king sent his mandate to the archbishop of York, the bishop of Carlisle, and William de Cantilupe, requiring them that they should earnestly persuade this opulent widow to take John de Plessets for her 2nd husband. Nay, so much did he desire the union that, upon Christmas day in the same year, being then at Bordeaux, he granted to John Plessets, by patent, the marriage of this Margery in case he could procure her consent; and if not, that then he should have the fine which the lady would incur by marrying with the king's license. This course of the king's however, prevailed, and his favourite obtained the hand of Margery de Newburgh, Countess of Warwick, and widow of John Mareschal, styled Earl of Warwick.
De Plessets was subsequently constituted constable of the Tower of London, but not by the title of Earl of Warwick, not did he assume that dignity for some time afterwards. He did, however, eventually assume it, under a clause in a fine levied in the 31st Henry III [1247], whereby William Mauduit, and Alice, his wife, did, as much as in them lay, confer the earldom upon him for life, so that, if he outlived the countess, his wife, he should not be forced to lay it aside. In the August ensuing, the King, granting to him license to fell oaks in the forest of Dene, styles him Earl of Warwick, and thenceforward he bore that dignity. His lordship was appointed in four years afterwards one of the justices itinerant to sit at the Tower for hearing and determining such pleas as concerned the city of London; and at the breaking out of the contest between Henry and the barons, he was constituted sheriff of the cos. Warwick and Leicester; but he lived not to see the issue of those troubles, for, falling sick in the beginning of the month of February, 1263, he d. before its expiration.
His lordship left issue by his first wife but none by the Countess of Warwick. Lady Warwick survived her husband but a short time when the Earldom of Warwick and the great inheritance of the Newburghs reverted to her cousin, Waleran de Newburgh, son of her aunt, Lady Alice Mauduit. By his first wife, Christian, dau. and heir of Hugh de Sandford, he had issue, a son and heir, Hugh de Plessets. [Sir Bernard Burke, Dormant, Abeyant, Forfeited and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 439, Plessets, or Plessetis, Earl of Warwick, Baron Plessets, and p. 399-400, Newburgh, Earls of Warwick]