His choice for a diplomatic career was inspired by his father. He joined the Dutch Diplomatic Service in 1938 and, after a two-year assignment at the Private Office of the Foreign Minister, was appointed as attaché in Bern (Switzerland) in 1940. In late 1941, he moved to Lisbon, Portugal. In both countries, he was involved in assistance to Dutch refugees, political espionage and counterintelligence. In 1943, he was transferred to the Dutch embassy in London. Ambassador Edgar Michiels van Verduynen discovered Luns's great affinity for the political element in international affairs and entrusted him with important files on Germany, which Luns handled with great skill.