Wellesley accompanied Charles Arbuthnot to Constantinople in 1805 and his uncle Richard Colley Wellesley, Marquess Wellesley, on his embassy to Spain in 1809. His father was informed by the latter, 30 Oct. 1809, ‘William is very diligent and I think you will find him improved. I have no doubt that he will listen to my advice. I shall bring him home with me.’ His family did not find him improved, but approved his courtship of the heiress Miss Tylney Long; he had ‘a real longing for her large dowry’ and, despite the rival attractions of the Duke of Clarence, she graduated from amusement to admiration for his perseverance. They were married and his name was elongated to the point of ridicule in March 1812. His uncle’s political fortunes had slumped and his wife’s trustees prevented him from placing her ‘£39,000 a year besides £300,000 in money’ at the disposal of the Wellesley squad’s electoral ambitions. The landed estates, too, were safeguarded by entail on their eldest son. Wellesley himself, now possessed of significant interests in the counties of Essex and Wiltshire, began to cut a figure in the former, entertaining lavishly at Wanstead.
Wellesley at first followed his uncle’s line in politics. He voted with the minority against the vice-chancellor bill, 11 Feb. 1813, and supported Catholic relief throughout that session. At the end of it his uncle’s squad was disbanded, partly at the instigation of his father, who in the following year became master of the Mint with a seat in the cabinet. Wellesley explained his conversion to ministers by reference to their foreign policy, in particular to their support of his uncle Wellington’s campaign in the Peninsula. He was not interested in a place, or in ‘foreign employment’, though he attended the congress at Vienna. Meanwhile he had spoken, as a protectionist and select committeeman in the Corn Law debates, 17 May, 6 June 1814 and 17 Feb. 1815. By his own account he differed from Castlereagh as to the settlement of Europe and ‘withheld from him my support of his general treaty’.
There is no sign of Wellesley’s attending the House in the session of 1818. On 25 Feb. he announced his candidature for Wiltshire, with a view to restoring the representation to his late father-in-law’s family: this was called by his opponents ‘the Long succession of Members’. He insisted that he was an Englishman born and bred and promised to reside in the county; denied that he used Treasury influence, though the son of a cabinet minister; denied that he was heir to a peerage (which he became a few years later); denied that he held place or pension, for he had refused both, and insisted on his political independence, for which he provided chapter and verse. The Marquess of Lansdowne reported of his success:
Wellesley Pole (fool as he passes for) has shown himself as great a canvasser as his uncle is a general, and without ever having given ... a vote for the people, contrived to persuade the people of Wiltshire who are not wiser than the people anywhere else that he was quite their man.
Ridiculed in the squibs as ‘the Dandy’, he defeated a Wiltshire squire for second place: but he was said to be ‘done for’ before his election and had borrowed £32,000 at 16 per cent interest to finance it.
On the road to ruin, he played a confident part in the Parliament of 1818, displaying his vaunted independence. He was opposed to further alteration in the Corn Laws, 22 Jan. 1819. He voted with the minority on the Westminster hustings bill, 3 Feb., and next day, in his longest speech to date, contested the proposals for the Windsor establishment. In the interests of public economy, he asked why the Duke of York (already commander-in-chief of the army) should receive the same grant as custos of the King as the Queen had received in 1812. When the grant was fixed at £10,000, he expressed a wish ‘to stand between the people and an unfair and exorbitant grant’ and voted with opposition, 22, 25 Feb. He ridiculed inadequate attempts to revise the Poor Laws, 9 Feb. 1819. Subsequently his attendance fell off and his tone was increasingly friendly to ministers. On 18 Mar. he accused the opposition of seeking popularity in opposing the Admiralty establishment. He approved the poor rates misapplication bill, 17 May. He voted against Tierney’s censure motion, 18 May, and for the foreign enlistment bill, which he defended, 21 June. As an ardent field sportsman, he had something to say about the reform of the Game Laws, admittedly ‘absurd and odious’, and approved an attempt to amend them, 19 Mar., 14 May. Next session he opposed Althorp’s motion for inquiry into the state of the country, as it would do no good, 30 Nov. 1819, defended the conduct of the sheriff of Wiltshire in refusing to call a county meeting, 2 Dec., and upheld the necessity of the seditious meetings prevention bill, 13 Dec.
Wellesley wished to stand again, but could not afford another contest for the county in 1820; nor, according to Lord Lansdowne, would he have stood ‘the smallest chance’. After ‘subsequent scandals’ he died ‘quite a beggar’, 1 July 1857.