Wellesley's diplomatic career began in 1791 when he was appointed attaché to the British embassy at The Hague. The next year, he became Secretary of Legation in Stockholm. In 1791 he exchanged into the 1st Foot Guards and in 1793 he purchased a Lieutenancy. In 1794, while on a trip home from Lisbon with his sister Anne, he was captured by the French, and remained in prison during the height of the terror, escaping only in 1795. In the latter year he sat for Trim in the Irish House of Commons. At the 1807 general election he was elected to the House of Commons of the United Kingdom as a Member of Parliament both for Athlone in Ireland and for Eye in England. He chose to sit for Eye, and held the seat until his resignation in 1809 by taking the Chiltern Hundreds. In 1797 Wellesley accompanied Lord Malmesbury as secretary on his unsuccessful mission to negotiate peace with the French at Lille. Later that year, he travelled to India, where he became private secretary to his oldest brother, Lord Mornington, the new governor-general. He was in India between 1797 and 1799, and again from 1801 to 1802, and was a useful assistant to his brother in a variety of diplomatic capacities, negotiating treaties with Mysore and Oudh. In 1802 he returned to Europe, and the next year married Lady Charlotte Cadogan, by whom he had three sons and a daughter before she abandoned him in 1809, running off with Lord Paget, a talented cavalry officer. His wife divorced him in Scotland in 1810. Although, at first, Paget's career was badly affected by the affair, as he was later unable to serve under Wellesley's brother Wellington in the Peninsular Campaign due to the bad blood, Paget later distinguished himself under Wellington at Waterloo. In 1809 Wellesley became the British envoy to Spain - his eldest brother, by now Marquess Wellesley, was now Foreign Secretary, while his brother Arthur was the commander of the Anglo-Portuguese forces in the Iberian Peninsula. Together, the three brothers helped to make the Peninsular campaign a success, and in 1812 Wellesley was knighted. He remained Ambassador to Spain until 1821, but found time to marry again, this time to Lady Georgiana Cecil, daughter of the Marquess of Salisbury. In 1823, Wellesley became Ambassador to Austria, where he remained until 1831. Although he was close acquaintances with Foreign Secretary George Canning, who had asked Wellesley to serve as his second in his duel with Lord Castlereagh, Wellesley felt that Canning did not appreciate his services, feeling him to be too conciliatory. In January 1828 Wellesley was created Baron Cowley, of Wellesley in the County of Somerset, due to his brother Wellington's influence with the prime minister, Lord Goderich. His final diplomatic service was in Paris, where he served as British ambassador to France during Robert Peel's administrations in 1835 and 1841–1846. In 1846 Cowley retired, but remained in Paris, where he died the next year.
