He was educated at Eton and Brasenose College, Oxford.
He served as British Ambassador to France between 1852 and 1867. He was a nephew of the 1st Duke of Wellington and the 1st Marquess Wellesley. Wellesley entered the diplomatic service in 1824, receiving his first important appointment in 1848, when he became Minister Plenipotentiary to the Swiss Cantons.
In July 1848 he was sent on special mission to the provisional central power of Germany in Frankfurt.
This was followed in June 1851 by his appointment as Envoy Extraordinary and Minister Plenipotentiary to the reinstated diet of the German Confederation, a position which he only held for a short time, as he was chosen in 1852 to succeed Lord Normanby as the British Ambassador in Paris. Lord Cowley, as Wellesley had become on his father"s death in 1847, held this important post for fifteen years, and the story of his diplomatic life in Paris cannot be separated from the general history of England and France.
As Minister during the greater part of the reign of Napoleon III, he conducted the delicate negotiations between the two countries during the time of those eastern complications which preceded and followed the Crimean War, and also during the excitement and unrest produced by the attempt made in 1858 by Felice Orsini to assassinate the Emperor of the French. While his diplomatic skill was no less in evidence during the war between France and Austria and the subsequent course of events in Italy.
In 1857 he was created Viscount Dangan, in the County of Meath, and Earl Cowley.
He was further honoured in 1866 when he was made a Knight of the Garter. Having assisted Richard Cobden to conclude the commercial treaty between Great Britain and France in 1860, he retired as ambassador in 1867. In 1863 Cowley inherited the former Long family estate of Draycot Cerne in Wiltshire from his kinsman the 5th Earl of Mornington, and he lived in retirement until his death on 15 July 1884.