In Paris, on Saturday, 3 September 1870, as news of the humiliating defeat of the French by the invading Prussian army at Sedan spread throughout the capital, Dr Adrien Proust, a middle-aged Catholic bachelor, a grocer's son originally from the small provincial town of Illiers, married Jeanne Weil, the Jewish daughter of a wealthy Parisian family. At twenty-one, the beautiful, dark-haired woman was fifteen years younger than the bridegroom. No one knows how they met, but it is likely they were introduced at a government sponsored event or social gathering. Adrien had recently risen to the top ranks in public health administration and Jeanne's family had many connections in official circles.

Marcel was born the following July at Uncle Louis Weil’s estate at Auteuil where Jeanne’s family usually spent the summer months. The house, built of quarrystones, was large, with spacious rooms, including a drawing room with a grand piano and a billiard room where the family sometimes slept to keep cool during heat waves. In fine weather Louis and his guests enjoyed the large garden with a pond surrounded by hawthorn trees, whose blossoms Marcel was also to admire in his other uncle, Jules Amiot’s garden in Illiers.