Robert Stewart the younger took the courtesy title of Viscount Castlereagh in 1796 when his father was promoted to the rank of earl. Thus, is generally known to history as Lord Castlereagh. The title of viscount was not officially inherited, but he became briefly the 2nd Marquess of Londonderry in the peerage of Ireland on the death of his father in 1821.
After serving in various junior positions in the Pitt and Addington governments, Castlereagh became Secretary of State for War and the Colonies in the Duke of Portland's administration in 1807. In that role he became involved in disputes with Foreign Secretary George Canning over the failure of the Walcheren Expedition, and the two fought a duel late in 1809. This forced both of their resignations from the government.
Three years later, in 1812, Castlereagh returned to the government, this time as Foreign Secretary, a role in which he served for the next ten years. In this role he was instrumental in negotiating what has become known as a Quadruple alliance between United Kindom, Austria, Russia and Prussia at Chaumont in March 1814, in the negotiation of the Treaty of Paris that brought peace with France, and at the Congress of Vienna. At the Congress of Vienna, Castlereagh proposed and architected a form of collective security for Europe, then called a Congress system. According to the Congress system main signatory powers were to meet periodically (every two years or so) and collectively manage European Affairs. The following ten years saw 5 European Congresses where disputes were resolved with a diminishing degree of effectiveness. Finally, by 1822, the whole system had collapsed because of the unreconcilable differences of opinion between United Kindom, Austia and Russia, and because of the lack of support for the Congress system in British public opinion.
In the years 1812 to 1822, Castlereagh continued to competently manage Britain's foreign policy, generally pursuing a policy of continental engagement uncharacteristic of British foreign policy in the nineteenth century.
Castlereagh was not known to be an effective public speaker and his diplomatic presentation style was a times obstruse. He did however enjoyed a great reputation for integrity, consistency and good will, which was perhaps unmatched by any diplomat of his time. His view on foreign policy were, unfortunately, ahead of his time and his country's insular world view.
Despite his many achievements, Castlereagh was extremely unpopular within the country due to his supposed reactionism abroad, and his support at home for the repressive measures of Home Secretary Lord Sidmouth. For this reason, Castlereagh is (among other from Lord Liverpool's cabinet) immortalised in Shellley's poem The Mask of Anarchy, a poem heavily critical of, and inspired by the Peterloo Massacre: