Claude Marc Boucherville de Baissac was born in Curepipe, Mauritius in 1907. His parents were British aristocratic landowners, who sent him to be educated at the Lycée Henri-IV in Paris. He worked for a mica mining company in Madagascar but later returned to France, where he went into advertising. After the German invasion he fled Paris for the south, walking across the Pyrenees, and spent seven months locked up in Spanish jails before he eventually reached the UK.

In March 1942 he was interviewed by SOE and accepted for training, joining the same group of students as Harry Peulevé, Francis Suttill and Roger Landes. Like all recruits, he was also given a false name ('Clement Bastable') to maintain his anonymity. De Baissac was soon recognised as one of F Section's most difficult characters, and the instructors quickly noticed his volatile nature and particularly his stubborn single-mindedness, but he did well in the preliminary parts of the course and was recommended for the role of circuit organiser. In July 1942 he and Peulevé parachuted near Nîmes to begin the SCIENTIST circuit in the Bordeaux area, but they were dropped too low and both suffered injuries. Peuleve was unable to walk, so de Baissac limped away alone to make contact with a resistance group in Cannes.

After recovering from his sprained ankle, he journeyed to Bordeaux in September, and began creating small resistance cells which successfully sabotaged naval and other key targets in the area. His sister, Lise de Baissac, who had also been recruited by F Section, had arrived in France at the same time to establish herself at Poitiers, acting as a liaison officer between SCIENTIST and circuits in Paris. De Baissac was able to send extensive intelligence back to Baker Street, and gave a detailed report when he returned to London in March 1943.

Just prior to leaving France, de Baissac had been introduced to André Grandclément, who was a senior member of the right-wing OCM resistance movement in south-western France. Grandclément had thousands of men at his disposal, and after dropping back into France in May, de Baissac spent most of his time arming and training them in guerrilla warfare, believing that an Allied invasion might be launched later that year. During the summer, the RAF dropped huge quantities of weapons and explosives to this new secret army, whose numbers soon swelled to around 20,000 men. In August de Baissac was recalled to London again (accompanied by sister Lise) for more talks, leaving his wireless operator, Roger Landes, in charge. He also left behind Mary Herbert, a courier for SCIENTIST who was now secretly carrying his child.

De Baissac had every intention of continuing his work in Bordeaux, but in September Grandclément was arrested and turned by the Germans, and the circuit soon began to collapse. There was no question of de Baissac returning to the area, so a new incarnation of SCIENTIST was planned to begin in Normandy. Arriving in February 1944, he soon set up in Chartres and was rejoined by sister Lise in April. He developed several sub-circuits, which successfully cut German lines of communication, destroyed several hundred enemy vehicles and fed back valuable intelligence to aid the Allied landings in June. In November 1944 de Baissac married Mary Herbert, who had given birth to their daughter, Claudine, nearly a year before: despite arrest and imprisonment by the Gestapo in Poitiers, she had managed to maintain her cover story and went into hiding with Claudine after her release.

For his actions, de Baissac was awarded the Distinguished Service Order and Bar, the Croix de Guerre and was made a Chevalier of the Légion d'Honneur. After leaving SOE he worked in Germany with the Allied Control Commission in Wuppertal, then became a director for a mining company in West Africa. He remarried in 1964, and later retired to Aix-en-Provence, where he died in 1974.