Guérisse was born in Brussels, and qualified in medicine at the Université Libre de Bruxelles before joining the Belgian Army. Guérisse was Médecin-Capitaine with a Belgian cavalry regiment during their eighteen-day campaign in May 1940. He managed to escape to England through Dunkirk. At Gibraltar, he joined the crew of a former French merchant ship, "Rhin", which had been renamed to serve in the Mediterranean on clandestine missions. He secured entry into the British Royal Navy and was commissioned as Lieutenant Commander Patrick Albert O'Leary RNVR of French-Canadian origin. The "Canadian" identity attempted to explain his not-quite British accent in English, and his not-quite French accent in French, in order to protect his family in occupied Belgium if he was captured. He had a six-week undercover training session with Naval Intelligence. Until April 1941, he was serving mainly as a conducting officer, escorting agents ashore in small boats through the surf, whilst the large vessel lay some distance offshore. This was skilled work, exposed to physical dangers from the sea-conditions and operational dangers from the Vichy security services.