In 1797 he was elected to parliament for the notorious rotten borough of Old Sarum, where he succeeded Richard Wellesley, 2nd Earl of Mornington.

After the shortlived Peace of Amiens broke down in 1803, Williams-Wynn overcame the apathy and the county rapidly two formed Troops of Yeomanry cavalry by August 1803.

[1][2] In 1806 he was appointed Under-Secretary of State for the Home Department in the Ministry of All the Talents led by his uncle Lord Grenville.

During the late 1810s, Williams-Wynn was the leader of a group of MPs that tried to establish a third party in the House of Commons, acting on behalf of his cousin Lord Buckingham.

In January 1822 Williams-Wynn was admitted to the Privy Council and appointed President of the Board of Control, with a seat in the cabinet, in the Tory government of the Earl of Liverpool.

This drove him into opposition, and when the Whigs came to power in November 1830 under Lord Grey, Williams-Wynn was appointed Secretary at War, although without a seat in the cabinet.

In 1834 the Tories returned to office under Sir Robert Peel, and Wynn was appointed Chancellor of the Duchy of Lancaster, but again was not a member of the cabinet.