The Mostyns claimed descent from Tudur Trevor, a tenth-century prince of Powys, and may have been stewards to the princes of Powys Fadog in the final decades of Welsh rule. They were, however, of little significance in the century following the Edwardian conquest, though one married a sister of Owen Glyndŵr, and their estates were briefly forfeited during the latter’s rebellion of 1400. Thereafter, they acquired extensive estates in Anglesey, Flintshire and Creuddyn in north-eastern Caernarvonshire through marriage. They supported the claims of their relatives Jasper and Henry Tudor to the English throne, but tradition states that Henry’s offer of a place at Court after the battle of Bosworth was declined.
Although they remained in Wales, the family did not escape the Anglicizing influences of the Tudor period. Thomas and Peter Mostyn† adopted the surname of Mostyn (the family’s principal residence) in the 1530s, and family members occasionally served as knights for Flintshire thereafter. The subject of this biography, and his step-brother Piers Griffith of Penrhyn, were among the first members of the family to be given an English education, and Mostyn later chose an English tutor for his eldest son Thomas, while several of his younger children were schooled at Hawarden ‘in respect of the English tongue’.
Mostyn’s English education may have been intended as the foundation for a career in the Church or the law, but the death of his elder brother William in about 1586 left him as heir to the estate. He apparently took control of the Mostyn coalmines during his father’s lifetime; these yielded a lucrative income of £700 per annum, which he used to buy lands worth £300 a year.
Mostyn was fortunate in having an independent income, as he had a stormy relationship with his father. The quarrel probably began in May 1600, when Mostyn was pressured to assign the family’s Gloddaith estate - part of which had earlier been promised to his own wife - to his new stepmother, Dame Katherine, as a jointure. However, in 1608, when Sir Thomas and his wife separated, Mostyn helped to negotiate the complicated arrangements for the maintenance of all parties.
Mostyn showed little interest in politics, but considerations of family honour probably obliged him to stand for the knighthood of the shire in December 1620, the first possible opportunity after his succession to the family estates. While apparently returned unopposed for Flintshire, his brother-in-law Sir Richard Wynn was challenged for the Caernarvonshire seat by John Griffith III*. Wynn’s supporters suggested that Mostyn, whose Gloddaith estates gave him considerable influence in the shire, should ‘solicit those that are either neuters, or inclining to the adverse side’, and hoped that his presence at the election would sway some of Griffith’s supporters. However, it was apparently Mostyn who was persuaded to change his mind, backing John Griffith in preference to Griffith Jones of Castellmarch, the compromise candidate put forward by Wynn’s supporters at the last moment when it became clear that they were facing defeat.
One consequence of Bacon’s fall was the promotion of Wynn’s relative John Williams to the keepership of the great seal. Sir Richard Wynn quickly secured Mostyn’s second son a place in the Williams’s household, for which favour Sir Roger sent Williams two silver flagons. Mostyn was tipped for inclusion on the commission of investigation sent to Ireland in 1622 under (Sir) William Jones I*, but Sir Richard Wynn claimed that his name had been dropped because ‘my lord [keeper] believed the employment would not have pleased him’. Williams apparently wished to gratify Mostyn in order to match his widowed sister with Mostyn’s heir, a plan to which Sir Roger was tactfully but vehemently opposed, ‘by reason (as he said) of the unequality of years between his son and her only, and not out of any other respect at all’; the lady eventually married (Sir) Peter Mutton*.
Mostyn showed no desire to stand for re-election to Parliament in 1624, when he swiftly pledged his support to his nephew Sir John Hanmer*. This frustrated the Wynns’ plans to promote Mostyn’s heir, Sir Thomas Mostyn, for the Flintshire seat, but Mostyn protested that ‘I may not with my credit leave him [Hanmer] though it were for the dearest I have, having so far engaged my word unto him long since’. Though Wynn had doubtless informed him that Sir Thomas was backed by his father-in-law, justice (Sir) James Whitelocke*, Mostyn claimed that if Whitelocke genuinely agreed, ‘he would have written unto me, or procured a burgess-ship in some other place’.
Mostyn was less opposed to the idea of his second son John standing for election in Anglesey, where he held a modest estate in 1624, apparently reasoning that his status as a younger son meant that ‘he hath little to lose, whatsoever fall thereof’. Mostyn wrote a lukewarm letter of recommendation to the local magnate Richard Bulkeley*, with the proviso that ‘if it may not be had without any contesting with any country gentleman, I hold it not worth the having’. The bearer of the letter, Mostyn’s brother-in-law Owen Wynn, carried the day by dint of vigorous canvassing, which intimidated two local candidates, Sir Sackville Trevor* and Rowland Whyte, into withdrawing before the the election.
Comparatively little is known about Mostyn’s life after 1627, when Sir John Wynn’s death deprived him of his most regular correspondent. He remained active in local administration, though he was reported to be ‘ancient and not able to travel’ in 1634, and he presumably supported the return of his son John Mostyn for Flintshire at both of the elections of 1640. Ever cautious, he drafted his will on the opening day of the Long Parliament, which recited provisions already made for his wife and younger sons. He died on 18 Aug. 1642, and was buried two weeks later, ‘with great pomp’. He was succeeded by his grandson Sir Roger, an active royalist during the Civil War, who acquired a baronetcy after the Restoration. His descendants regularly represented Flintshire in Parliament until the failure of the male line in 1831.