He remained in the House of Commons until 1806, when he was appointed to the Privy Council, and Ambassador to the court of St. Petersburg until 1807; additionally, he was Lord Lieutenant of Lanarkshire from 1802 to 1852.

An obituary notice states that "timidity and variableness of temperament prevented his rendering much service to, or being much relied on by his party ... With a great predisposition to over-estimate the importance of ancient birth ... he well deserved to be considered the proudest man in England."

Lord Lamington, in The Days of the Dandies, wrote of him that 'never was such a magnifico as the 10th Duke, the Ambassador to the Empress Catherine; when I knew him he was very old, but held himself straight as any grenadier.

Lady Stafford in letters to her son mentioned 'his great Coat, long Queue, and Fingers cover'd with gold Rings', and his foreign appearance.

In accordance with his wishes, Hamilton's body was mummified after his death and placed in a sarcophagus of the Ptolemaic period that he had originally acquired in Paris in 1836 ostensibly for the British Museum.