The Mostyn children were brought up at Mostyn and Gwydir with William*, Ellis and Henry Wynn*, the youngest of their uncles. While Sir Thomas Mostyn† was rarely on good terms with his son Sir Roger, he doted upon John, praising his infant grandson as ‘my servingman and best in my house, saving that he is of the humour that other servingmen be and giveth me warning every day that he will go to London to seek service’. In his will, which pointedly ignored Sir Roger and the rest of his children, Sir Thomas bequeathed John his ‘best chain of gold’.
It was presumably Sir John Wynn who obtained Mostyn a place in the household of his relative and former protégé John Williams, dean of Westminster, shortly before the latter was appointed lord keeper and bishop of Lincoln in 1621. By the end of the year, Mostyn’s uncle Owen Wynn noted that ‘we are all bound for his [Williams’s] love to Jack Mostyn and my brother William [Wynn] who[m] he doth advise and direct as though they were his own children’.
In December 1623, when the failure of the Spanish Match led to the calling of a fresh Parliament, Sir Roger Mostyn received letters ‘from both my sons Thomas and John, and both tended to one end, for a place in Parliament for this shire’. While the Flintshire gentry had already settled upon their candidates, Sir Roger put John forward for the knighthood of the shire of Anglesey, where he had just granted his son a reversionary interest in the family’s rarely visited estate of 660 acres.
Mostyn left little trace on the records of the Parliament, though if the allegations of miscarriages of justice which were being levelled at lord keeper Williams had resulted in impeachment proceedings he would undoubtedly have been expected to help in his master’s defence.
Mostyn was not actively involved in the debates on the Spanish Match, which his master strongly supported, but his family’s papers contain a copy of the duke of Buckingham’s ‘Relation’ of the breakdown of diplomatic negotiations, delivered on 24 Feb. 1624. Mostyn probably obtained a copy from Williams, who had made the official report of Buckingham’s speech to the Lords on 27 Feb., and his text included additional letters concerning the Match from the 1st earl of Bristol (Sir John Digby*), which took a much more negative view of Buckingham’s actions than the duke’s own explanation.
Mostyn returned to Wales in the summer of 1624, when Owen Wynn advised ‘that it is expected that Jack Mostyn should in person at Beaumaris give the gentlemen of that country thanks [for his election] this assizes and remit the mise [wages for his service as MP] ... else another time we shall be shaken off, when the like occasion is offered’.
Mostyn sacrificed all hope of personal advancement by remaining in Williams’s service after the latter’s dismissal from the keepership at the end of October 1625.
Mostyn was returned to both the Short and Long Parliaments for Flintshire; he left little trace on the records of either Parliament, and was licensed to return to Wales on 10 Nov. 1642, where his father’s recent death had left him the senior surviving member of the family.
