1844 he held the command of the Montgomeryshire yeomanry cavalry. The three brothers Wynn are depicted in Gillray's caricature of ‘A Welch Tandem’ (21 Jan. 1801), and on 19 May 1806 his elder brother, Sir Watkin Wynn, and he, figured in the same artist's caricature of ‘The Bear [C. J. Fox] and his Leader’ [Lord Grenville]. In it they were called ‘Bubble and Squeak, a Duet,’ nicknames which had been given to them through the peculiarity of their voices (Wright and Evans, Gillray Caricatures, pp. 269, 463). From 19 Feb. 1806 to October 1807 he was under-secretary of state for the home department in the administration of ‘all the talents,’ which was presided over by his uncle, Lord Grenville.
Wynn was fond of parliamentary life and took an active part in debate, being considered a great authority on points of procedure. He was proposed for speaker on 2 June 1817, and in the opinion of Sir Samuel Romilly was eminently qualified for the post, as he had ‘by long attention to the subject made himself completely master of the law of parliament and the forms of parliamentary proceeding’ (Memoirs, iii. 296–297). But Manners-Sutton was supported by the government, and won by 312 votes to 152. Canning said that the only objection to Wynn was that ‘one would be sometimes tempted to say Mr. Squeaker’ (Moore, Diary, v. 273).
During 1818 and 1819 Wynn endeavoured, as leader of the members acting in the interest of his relative, the Marquis of Buckingham, to form a third party in the House of Commons, but some of the little party of politicians thought that he leaned too much to the side of the whigs. In 1819 he was on the civil list committee, and during 1820 he strongly objected to the conduct of the king and his ministers towards Queen Caroline. When these troubles were over, the support of Lord Buckingham's adherents was secured by the tory ministry. From January 1822 to February 1828 Wynn held the post of president of the board of control with a seat in the cabinet, and on 17 Jan. 1822 he was sworn of the privy council. In September 1822 Canning, who liked him not, desired his transfer to some other office to make room for Huskisson. There were differences between them in the following year, and in 1824 Canning called him ‘the worst man of business that I ever met’ (Canning Corresp. i. 201). Nevertheless he remained in office for six years, even through the brief administrations of Canning and Lord Goderich. When the Duke of Wellington formed his cabinet in 1828 the Duke of Buckingham, who had long pressed his claims to high office, thought that he, and not Wynn, should have a place in it, and Wellington thereupon ejected Wynn, as Southey said, ‘with a want of courtesy, of respect, and of feeling.’ But even Southey had heard that Wynn was ‘one of the most impracticable persons to deal with, taking crotchets in his head, and holding to them with invincible pertinacity’ (Letters, ed. Warter, iv. 132–3).
After his loss of office Wynn was drawn into opposition. He supported O'Connell's claims to sit for the county of Clare, and he voted for Sir Henry Parnell's motion on the civil list which brought about the downfall of the Wellington ministry. In the succeeding administration of Lord Grey he was secretary at war from November 1830 to April 1831, but without a place in the cabinet; and he was also a member of the board of control. He did not approve of Lord John Russell's disfranchisement proposals in the Reform Bill, and, although he voted for the second reading of the measure, he supported General Gascoyne's amendment, when the whig government were defeated by 299 votes to 291.
In 1831 Wynn was active on the commission for inquiry into the public records, and in Sir Robert Peel's short ministry of December 1834 to April 1835 he held the office of chancellor of the duchy of Lancaster, but was not called into the cabinet. On 3 Feb. 1835 he was appointed a commissioner to inquire into the state of the dioceses in England and Wales. He is said to have thrice refused the post of governor-general of India. Although he sat in parliament until 1850, he was not again offered office, and he gradually withdrew from public affairs, preferring to dwell at his pleasant country seat, with its ‘noble terraces,’ Llangedwin in Denbighshire (Southey, Life and Corresp. iv. 354). He was the first president of the Royal Asiatic Society, taking a leading part in its proceedings from its foundation in 1823, but he resigned the position in 1841. He was elected F.S.A. on 9 Jan. 1800.
Wynn died at 20 Grafton Street, London, on 2 Sept. 1850, aged 74, and was buried by the side of his wife and son in a vault of St. George's Chapel, Bayswater. He married, on 9 April 1806, Mary (d. 4 June 1838), eldest daughter of Sir Foster Cunliffe, bart., of Acton Park, Denbighshire, and had issue two sons and five daughters. The eldest daughter, Charlotte Williams Wynn [q. v.], is noticed separately. Sidney, the fourth daughter, married, on 12 Dec. 1844, Sir Francis Hastings Doyle, who describes these sisters ‘as women of a very noble type.’
