Mostyn was one of Wales’s largest landowners, with estates and influence in Caernarvonshire, Flintshire, Merioneth and elsewhere. His life had long been governed by his passion for the chase, which he pursued in the company of his brother-in-law Sir Edward Pryce Lloyd* and their kinsman, the hunting parson Griffith Lloyd, in North Wales and the Wirral, and as master of the Bicester Hounds in Gloucestershire, Oxfordshire and Warwickshire.
Mostyn apparently failed to sign the requisition for a Flintshire county meeting to mark the death of George III and remained in London until shortly before his election at Flint, 16 Mar. 1820. His addresses contained no statements of policy, but his Whiggism and support for Catholic relief were well known. He afterwards headed the requisition for a meeting to petition for a St. Asaph enclosure bill, and announced a 25 per cent reduction in his rents.
He failed to attend the Flintshire agricultural distress meeting, 17 Apr., but he presented their petition for government action, 24 Apr., and voted to repeal the salt tax, 28 June 1822. He had recently divided for reductions in diplomatic expenditure, 16 May, to condemn the growing influence of the crown, 24 June, and for inquiry into the lord advocate’s treatment of the Scottish press, 25 June 1822.
Increasingly troubled by gout, Mostyn now rarely attended the Commons. He presented his constituents’ petitions for repeal of the Test Acts, 7 June 1827, 19 Feb. 1828, but is not known to have spoken or voted on the issue. He paired for Catholic relief, 12 May 1828, and presented petitions against restricting the circulation of one and two pound notes, 2 June, and for an increase in coroners’ allowances, 7 June 1828. Ill with rheumatism, he ordered Mostyn to be closed and returned to London in February 1829 to monitor the progress of Catholic emancipation, for which he voted, 30 Mar. He frequented the Welsh Club, attended the Commons, 24 Feb., 18 Mar., and presented, but refused to endorse Flintshire anti-Catholic petitions, 26 Feb. Citing ‘pressure of business’, he declined to attend anti-Catholic meetings, 20 Mar., and the Flintshire Ultras accordingly turned to his brother-in-law Sir Robert Williames Vaughan* for support.
Mostyn spent the summer of 1829 in London and at the races, often in the company of the sisters of William Lewis Hughes*: Mary, with whom his name was now linked, and Martha, the widow of his friend Cynric Lloyd, who later married Colonel Wyatt.
The Wellington ministry listed Mostyn among their ‘foes’, but he was absent from the division on the civil list which brought them down, 15 Nov. 1830. By February 1831, when Martha Wyatt protested that Mostyn had encouraged gossip that she had colluded in making her sister his resident mistress, he was too ill and dependent on opiates to consider going to the opera or to attend to parliamentary business, which then included the Holyhead roads bill.
