After the outbreak of World War II, Dewavrin was assigned to Norway in 1940 before he joined General Charles de Gaulle in Britain.

He received the rank of major, took charge of the Free French military intelligence unit Bureau Central de Renseignements et d'Action (BCRA) and took the codename "Colonel Passy".

Later in 1943, Dewavrin's organization was merged with the conventional secret service of the Free French Forces to form DGSS under Jacques Soustelle.

After the Normandy Invasion, Dewavrin became Chief of Staff to General Marie Pierre Koenig, the Commander of the French Forces of the Interior.

According to The Secret War by Max Hastings, a Soviet spy made a claim in a report to Moscow that Dewavrin had been recruited by Wilhelm Canaris to work for the Germans.