The early life of Daniel Cordier reads like something out of a John le Carré thriller: soldier, spy, resistance hero, art dealer. After parachuting into Nazi-occupied France in 1942 aged 22 on the orders of the exiled General Charles de Gaulle in London, Cordier worked with the leader of the French resistance, Jean Moulin.

Their mission – to unify the disparate underground network across the country – was vital to the war effort, but also sparked in Cordier a lifelong love of contemporary art as the pair criss-crossed France, posing as art dealers seeking paintings and collectable artefacts.

Now Sotheby’s in Paris is to auction off part of the collection which Cordier, 98, went on to amass over his lifetime. Olivier Fau, Sotheby’s senior director for contemporary art, says the paintings going under the hammer this month are a “time capsule back to another moment in time, another world”.

“Daniel Cordier was personal secretary to Jean Moulin, who was passionate about art all his life. Moulin would talk about paintings when speaking to other Resistance members to deceive the Germans. Art was the code,” Fau told the Observer.

“He passed on this passion, and after the war Cordier became an art dealer and began collecting postwar art. He had a collector’s spirit and didn’t like to sell his works. Now he has decided to sell a large number, which takes us back to a moment in time more than 60 years ago.”

Cordier was awarded the Grand Croix of the Légion d’Honneur, the highest decoration of the French state, earlier this year.

Cordier, who has described himself as a fierce monarchist and antisemite before France capitulated to the Germans, viewed Marshal Pétain’s declaration of the armistice agreement on 17 June 1940 as a disgraceful betrayal. It prompted Cordier and 16 likeminded friends to set off for London, landing in Falmouth seven days later. Frustrated at not being sent into action after military training, he transferred to the secret services and learned the art of sabotage, radio transmission and parachuting.

On landing back in the south of France in 1942, Cordier, codename Bip W, was ordered to make contact in Lyon with a man known only as “Rex” who was, in fact, Jean Moulin.

Moulin, already a fine art collector, had to find a cover for his resistance work and registered with the collaborationist Vichy government as a painter and art dealer. As the Germans closed in on the Resistance, Moulin bolstered his cover by opening a gallery in Nice and exhibited his own works as well as other artists in the region and masters including Dégas, Dufy and Matisse, presenting Cordier as his assistant.

When Moulin died after he was arrested by the Gestapo in 1943, Cordier continued to rally and organise Resistance fighters while dodging the Germans himself, finally fleeing over the Pyrenees where he was arrested and interned before being transferred back to London in May 1944.

Daniel Cordier’s Paris gallery a 8, Rue de Miromesnil in 1964. Photograph: WestImage - Art Digital Studio/Robert David/Sotheby's

After the war, Cordier opened a gallery in Paris – described by newspapers at the time as the “most original” in the French capital for its promotion of previously little-known contemporary artists.

“A work of art is not to ease but to awaken and hassle the mind,” Cordier declared. He continued collecting paintings, drawings, sculpturesand manuscripts and was a founder of the Musée National d’Art Moderne centred at the Pompidou Centre.

“The generous donations he has given to the national museum are perhaps the most important in France in the whole second half of the 20th century. In money terms, it amounts to more than all other donations during that period,” Fau said.

Cordier wrote a bestselling and award-winning novel, Alias Caracalla, based on his wartime experiences with Moulin. Today he is just one of five survivors out of the 1,038 heroes declared “Companions of the Liberation” after the war.

Among the key objects in the 400-lot Sotheby’s auction are works by Jean Dubuffet, Simon Hantaï, Henri Michaux, Jean Dewasne, Robert Mapplethorpe, Louise Nevelson and Roberto Matta.

Cordier said discovering the “still world of paintings” at the age of 24 had “turned my existence into an adventure”. Now living in Cannes, he told Le Monde earlier this year: “I’m an old but very, very happy old man”, describing his experience as not one life but “successive lives that were so different from one another”.Decorating Cordier in May this year, President Emmanuel Macron told him: “To be face to face with you is to find oneself immediately and compellingly face to face with history.”