In February 1939, Pétillon asked to be posted to the Belgian Congo and received a position as "chef de cabinet" for the Governor-General, Pierre Ryckmans. He was still in the Congo in May 1940 when Belgium was invaded by Germany. Despite being posted as De Vleeschauwer's "chef de cabinet" with the Belgian government in exile in London, Pétillon spent most of World War II in the Congo. After the Liberation of Belgium in September 1944, Pétillon played a major role in re-establishing relations between the colonial administration in Africa and the Ministry of the Colonies.