In 1910 Buchanan was appointed as the British Ambassador in Russia. He kept abreast of the political developments in Russia and met with some of the leading liberal reformists in the country. It has been suggested that this was secretly encouraged by the then Liberal government in London.
Buchanan was the ambassador at the time of the Russian Revolution in 1917 and he developed close relations with the liberal Provisional Government that transpired after the February revolution. However, after the events of the October revolution and the Bolsheviks ascendency to power he was widely criticised for the failure to ensure that Tsar Nicholas II and his family were evacuated from Russia before their execution by the Bolsheviks in 1918.
Buchanan's autobiography, My Mission to Russia and Other Diplomatic Memories, was published in 1923. He died in 1924.