Cudbert Thornhill was born in Tibet on 4th October 1883. His father was an army officer who was in charge of the British frontier station. Thornhill became fluent in a number of languages and later joined his father in the British Indian Army.
On the outbreak of the First World War Thornhill was recruited by Mansfield Cumming into MI6. He went to work for Lieutenant-Colonel Samuel Hoare, the head of the SIS station in Petrograd. Other members of the unit included Oswald Rayner, Stephen Alley and John Scale.
In 1916 Thornhill was made an assistant military attaché, controlling the collection of military intelligence. A journalist who met him in Russia during the war claimed that Thornhill was "one of the bravest men - and most silent - I have ever met." He added that Thornhill was "a calm dignified, silent man, almost detached in his bearing, until the moment came for quick action; then the iciness would erupt like a volcano." One of his agents was Arthur Ransome, the Russia correspondent of The Daily News.
In 1917 Thornhill and John Scale were sent to Romania: where they compiled assessments of how prepared Russian troops were for an attack by the German Army. Scale informed Mansfield Cumming that a "Russian offensive on this front should not be relied on. There is no confidence in success among men."
Thornhill returned to Petrograd during the Russian Revolution. According to one source Thornhill was "the hero of many exciting adventures in Petrograd during the revolution". He was forced to go into hiding but by January 1918, he was back in the capital reporting that the new Red Army was being formed from munitions workers because they were considered to be more intelligent than peasants.
Michael Smith, the author of Six: A History of Britain's Secret Intelligence Service (2010) argued that during the Russian Civil War: "Thornhill had been setting up agent networks across the north between Murmansk and the White Sea port of Kem to warn the British of any Bolshevik advances.... Thornhill was allocated as the force's chief intelligence officer, and a number of MI1c officers worked with him."
In 1939 Colonel Thornhill married Rachel Crowdy, Principal Commandant of Voluntary Aid Detachments in France and Belgium (1914 to 1919) and Chief of the Department of Opium Traffic and Social Issues Section of the League of Nations (1919 to 1931).
Cudbert Thornhill died in 1952.