On 10 May 1940 Germany invaded Luxembourg. Konsbruck accompanied Hereditary Grand Duke Jean, Princess Marie Gabrielle, and Princess Alix as they fled to France. Konsbruck served as "aide de camp" to Grand Duchess Charlotte during her exile in World War II. After arriving in Bordeaux, Guillaume Konsbruck and his family were granted transit visas from the Portuguese consul Aristides de Sousa Mendes, in June 1940. They travelled to Portugal along with the rest of the government and the Grand Ducal Family of Luxembourg. Guillaume, along with his wife Nelly, and their sons Guy and Carlo, followed the Grand Ducal family through Coimbra and Lisbon. They settled at Praia das Maçãs after the Grand Ducal family moved to Cascais. By August, the entire entourage had moved to Monte Estoril and the Konsbruck family stayed at Chalet Posser de Andrade with the Grand Ducal family. They left Portugal with the Grand Duchess Charlotte and the Dowager Grand Duchess Anne Marie on 3 October 1940 aboard the Pan Am Yankee Clipper headed for New York City.