In 1814 he was also appointed Ambassador to Austria, a post he held for nine years (1814–1823).
On 18 June 1814, to make him more acceptable in Vienna, Stewart was ennobled as Baron Stewart, of Stewart's Court and Ballylawn in County Donegal, by the Prince Regent.
In the same year, he received honorary degrees from Oxford and Cambridge, was admitted to the Privy Council, and was appointed a Lord of the Bedchamber to the King.

Lord Stewart, as he now was, attended the Congress of Vienna with his half-brother Lord Castlereagh as one of the British plenipotentiaries.
He was not well regarded as he made a spectacle of himself with his loutish behaviour, was apparently rather often inebriated, frequented prostitutes quite openly.
He earned himself the sobriquet of Lord Pumpernickel after a loutish character in a play in fashion.
In 1814 he was also appointed Ambassador to Austria, a post he held for nine years (1814–1823). On 18 June 1814, to make him more acceptable in Vienna, Stewart was ennobled as Baron Stewart, of Stewart's Court and Ballylawn in County Donegal, by the Prince Regent. In the same year, he received honorary degrees from Oxford and Cambridge, was admitted to the Privy Council, and was appointed a Lord of the Bedchamber to the King.
Lord Stewart, as he now was, attended the Congress of Vienna with his half-brother Lord Castlereagh as one of the British plenipotentiaries. He was not well regarded as he made a spectacle of himself with his loutish behaviour, was apparently rather often inebriated, frequented prostitutes quite openly. He earned himself the sobriquet of Lord Pumpernickel after a loutish character in a play in fashion.
Charles Vane, 3rd Marquess of Londonderry