Resnais was born in 1922 at Vannes in Brittany, where his father was a pharmacist. An only child, he was often ill with asthma in childhood, which led to his being withdrawn from school and educated at home. He was an eager reader, in a range that extended from classics to comic books, but from the age of 10 he became fascinated by films. For his twelfth birthday his parents gave him a Kodak 8mm camera with which he began to make his own short films, including a three-minute version of Fantômas. Around the age of 14, he discovered surrealism and through that an interest in the works of André Breton.
Visits to the theatre in Paris gave Resnais the desire to be an actor, and in 1939 he moved to Paris to become an assistant in Georges Pitoëff's company at the Théâtre des Mathurins. From 1940 to 1942 he studied acting in the Cours René-Simon (and one of his small jobs at this time was as an extra in the film Les Visiteurs du soir), but he then decided in 1943 to apply to the newly formed film school IDHEC to study film editing. The filmmaker Jean Grémillon was one of the teachers who had the most influence on him at that period.
Resnais left in 1945 to do his military service which took him to Germany and Austria with the occupying French forces, as well as making him a temporary member of a travelling theatre company, Les Arlequins. He returned to Paris in 1946 to start his career as a film editor, but also began making short films of his own. Finding himself to be a neighbour of the actor Gérard Philipe, he persuaded him to appear in a 16mm surrealist short, Schéma d'une identification (now lost). A more ambitious feature-length work, Ouvert pour cause d'inventaire, has also vanished without trace.