[1] Stuart joined the Diplomatic Service in 1801, and was immediately appointed as Secretary of Legation in Vienna, Holy Roman Empire, a post he held until 1804.

In December 1814 Henry Brougham described Stuart to Thomas Creevey:He is a plain man, of some prejudices, caring little for politics and of very good practical sense...

During Napoleon's Hundred Days, he left Paris and was in Brussels at the start of the Waterloo Campaign, where during his stay he attended the Duchess of Richmond's Ball.

[6] After the fall of Napoleon, he escorted the exiled French King Louis XVIII back to Paris, and became British Ambassador there until 1824.

In 1825 the Portuguese King John VI named Stuart his plenipotentiary with powers to negotiate and sign with Brazil a Treaty on the recognition of that country's independence.

[1] Lord Stuart de Rothesay married Lady Elizabeth Margaret, daughter of Philip Yorke, 3rd Earl of Hardwicke, on 6 February 1816.

[11] Stuart engaged William Donthorne, a founder member of the Royal Institute of British Architects, to design a new Highcliffe Castle.

An oriel window is central on the south east elevation, providing a vista across the landscaped gardens to a panorama of The Needles and Isle of Wight.