After spending his childhood in Quimper, Brittany, France, he enrolled in the Paris Colonial School, which he left in 1897 for an artistic career. On the Boulevard Voltaire, he shared a room with Pablo Picasso, who introduced him to Guillaume Apollinaire, who in turn introduced him to Georges Braque. He would become close friends with Jean Cocteau, Jean Hugo, Christopher Wood and Amedeo Modigliani, who painted his portrait in 1916. He also befriended and encouraged the artist Romanin, otherwise known as French politician and future Resistance leader Jean Moulin. Jacob, who had Jewish origins, claimed to have had a vision of Christ in 1909, and converted to Catholicism.