Percy Smythe, 6th Viscount Strangford

In 1806, he served as chargé d'affaires under the Earl of Rosslyn and the Earl of St Vincent, the Extraordinary Envoys of the United Kingdom to Portugal. In 1807, he was appointed British Envoy Extraordinary and Minister Plenipotentiary to Portugal under the reign of King George III.[4] In 1807, as Britain's envoy to Portugal, Lord Strangford coordinated the Portuguese royal family's flight from Portugal to Brazil. Lord Clinton, as he was known in Brazil, he arrived with the Royal Family in Salvador in January 1808 and soon they moved to Rio de Janeiro where they arrived on 8 March 1808.[5][6] Lord Clinton and the Brazilian accountant Dom Fernando José de Portugal had hard work to do in the Brazilian Imperial Palace. They had to raise the money moved from Portugal to Brazil under English escort. Their work was for thirty days. The tax service of 2% was according to the Prize Money (the law had been cancelled in 1803 and was re-edited in 1807).[7] They counted one hundred million Pounds and two million pounds in taxes. (In that year, with that money would be possible to buy two hundred million bags of coffee, nowadays it is U$20 billion). After that, the payment delayed fourteen years to be paid after the English recognizance of Brazilian Independence. That was the money Napoleon wanted to finance his war against England.[8] Napoleon said in his memoirs that Don John was the only one to trick him.[9][7]

He was British Envoy Extraordinary and Minister Plenipotentiary at the Court of Stockholm in Sweden from 1817 to 1820, during the reign of Charles XIII of Sweden and Charles XIV John of Sweden.[10]

The Levant Company nominated Lord Strangford and his appointment was confirmed in 1820 as the British Ambassador to the Ottoman Empire.[11] He was successful in his efforts to secure the consolidation of the new constitutional settlement between the Ottoman Empire and the Danubian Principalities which followed the revolution in Wallachia in 1821, to persuade the Ottomans to withdraw their troops from the Principalities, and to dissuade the Russian Empire from military intervention.[12]

As ambassador to the Sublime Porte, he had opportunities to assemble fragments of Greek sculpture. Among his collection of antiquities was the "Strangford Shield", a 3rd-century CE Roman marble that reproduces the shield of Athena Parthenos, Phidias' sculpture formerly in the Parthenon. The "Strangford Shield" is conserved in the British Museum. He left Turkey in 1824.

From 1825 to 1826, he served as British Ambassador Extraordinary and Plenipotentiary at St. Petersburg, Russia,[13] when he[14] was created Baron Penshurst, of Penshurst in the County of Kent, in the Peerage of the United Kingdom, enabling him to sit in the House of Lords.[15] His diplomatic career went into decline after he was caught falsifying dispatches to the British government and revealing confidential documents to the Austrian ambassador in St Petersburg.[5]

He was appointed Grand Cross of the Order of the Bath (GCB) in 1815 and Knight Grand Cross of the Royal Guelphic Order (GCH) in 1825. In February 1825, he was elected a Fellow of the Royal Society[21] He translated the Rimas of Luís de Camões in 1825.

A window in his family chapel in St. Mary's Church, Ashford, Kent, commemorates him, mentioning the monarchs whom he served and the countries to which he was dispatched.

Through his eldest son with Benham, he was the grandfather of Minnie Smythe (1872–1955), also a painter.[22]