By his father's death in 1781 he became 2nd Earl of Mornington, taking his seat in the Irish House of Lords. In 1784 he entered the English House of Commons as member for Beeraiston. Soon afterwards he was appointed a lord of the Treasury by Pitt. in 1793 he became a member of the Board of Control over Indian affairs; and, although he was best known by his speeches in defence of Pitt's foreign policy, he was gaining the acquaintance with Oriental affairs which made his rule over India so effective from the moment when, in 1797, he accepted the office of Governor-General.
Mornington seems to have caught Pitt's large political spirit during his intercourse with him from 1793 to 1797. That both had consciously formed the design of acquiring a great empire in India to compensate for the loss of the American colonies is not proved; but the rivalry with France, which in Europe placed England at the head of coalition after coalition against the French republic and empire, made Mornington's rule in India an epoch of enormous and rapid extension of English power. Clive won and Warren Hastings consolidated the British ascendancy in India, but Mornington extended it into an empire. On the voyage outwards he formed the design of annihilating French influence in the Deccan. Soon after his landing, in April 1798, he learnt that an alliance was being negotiated between Tippoo Sultan and the French republic. Mornington resolved to anticipate the action of the enemy, and ordered preparations for war. The first step was to effect the disbandment of the French troops entertained by the Nizam of Hyderabad. The invasion of Mysore followed in February 1799, and the campaign was brought to a rapid close by the capture of Seringapatam. In. 1803 the restoration of the peshwa proved the prelude to the Mahratta war against Sindh and the raja of Berar. The result of these wars and of the treaties which followed them was that French influence in. India was extinguished, that forty million people and ten millions of revenue were added to the British dominions, and that the powers of the Mahratta and all other princes were so reduced that England became the really dominant authority over all India. He found the East India Company a trading body, he left it an imperial power.
He was an excellent administrator, and sought to provide, by the foundation of the college of Fort William, for the training of a class of men adequate to the great work of governing India. In connection with this college he established the governor-generals office, to which civilians who had shown talent at the college were transferred, in order that they might learn something of the highest statesmanship in the immediate service of their chief. A free-trader, like Pitt, he endeavoured to remove some of the restrictions on the trade between England and India. Both the commercial policy of Wellesley and his educational projects brought him into hostility with the court of directors, and he more than once tendered his resignation, which, however, public necessities led him to postpone till the autumn of 1805. He reached England just in time to see Pitt before his death. He had been created a Peer of Great Britain in 1797, and in 1799 became Marquess Wellesley in the Peerage of Ireland.
On the fall of the coalition ministry in 1807 Wellesley was invited by George III to join the Duke of Portland's cabinet, but he declined, pending the discussion in parliament of certain charges brought against him in respect of his Indian administration. Resolutions condemning him for the abuse of power were moved in both the Lords and Commons, but defeated by large majorities. In 1809 Wellesley was appointed ambassador to Spain. He landed at Cadiz just after the Battle of Talavera, and endeavoured, but without success, to bring the Spanish government into effective co-operation with his brother, who, through the failure of his allies, had been compelled to retreat into Portugal.A few months later, after the duel between Canning and Castlereagh and the resignation of both, Wellesley accepted the post of Foreign Secretary in Perceval's cabinet.
He held this office until February 1812, when he retired, partly from dissatisfaction at the inadequate support given to Wellington by the ministry, but also because he had become convinced that the question of Catholic emancipation could no longer be kept in the background. From early life Wellesley had, unlike his brother, been an advocate of Catholic emancipation, and with the claim of the Irish Catholics to justice he henceforward identified himself. On Perceval's assassination he refused to join Lord Liverpool's administration, and he remained out of office till 1821, criticizing with severity the proceedings of the Congress of Vienna and the European settlement of 1814, which, while it reduced France to its ancient limits, left to the other great powers the territory that they had acquired by the Partitions of Poland and the destruction of Venice. He was one of the peers who signed the protest against the enactment of the Corn Laws in 1815.
In 1821 he was appointed Lord Lieutenant of Ireland. Catholic emancipation had now become an open question in the cabinet, and Wellesley's acceptance of the viceroyalty was believed in Ireland to herald the immediate settlement of the Catholic claims. The Orange faction was incensed by the firmness with which their excesses were now repressed, and Wellesley was on one occasion mobbed and insulted. But the hope of the Catholics still remained unfulfilled. Lord Liverpool died without having grappled with the problem. Canning in turn passed away; and on the assumption of office by Wellington, who was opposed to Catholic emancipation, his brother resigned the lord-lieutenancy. He had, however, the satisfaction of seeing the Catholic claims settled in the next year by the very statesmen who had declared against them. In 1833 he resumed the office of Lord Lieutenant under Earl Grey, but the ministry soon fell, and, with one short exception, Wellesley did not take any further part in official life. He died on 6 September 1842. He had no successor in the marquisate, but the earldom of Mornington and minor honours devolved on his brother William, Lord Maryborough, on the failure of whose issue in 1863 they fell to the 2nd Duke of Wellington.
He lived together with Hyacinth Gabrielle Roland, an actress (and by some accounts a prostitute) for many years. Her mother's husband was Pierre Roland, but she was said to be the daughter of Christopher Alexander Fagan. She bore him three sons and two daughters by Wellesley before he married her on 29 November 1794. Their daughter Anne was an ancestor of Queen Elizabeth the Queen Mother. His first wife died in 1816. On 29 October 1825 he married his second wife, Marianne (Caton) Patterson, whose mother Mary was the daughter of Charles Carroll of Carrollton, the last surviving signer of the Declaration of Independence. They had no children.
Original text of this article largely based on the 1911 Encyclopaedia Britannica.