Loiseau was born in Paris on 3 October 1865 into an affluent commercial family. He was largely self-taught and during his youth he worked with a decorator, painting the outskirts of Paris in his free time.
An inheritance from his grandmother provided him with a certain degree of financial comfort and independence that allowed him to concentrate fully on paining and to move to Montmartre. Here he met Maxime Maufra, with whom he remained friends for the rest of his life. He enrolled at the École des Arts Décoratifs for a year, the only training in colour and drawing which he received.
In 1888 Loiseau visited Pont-Aven with Maufra, where he met Henry Moret and Paul Gauguin.
After this first visit he made frequent trips to the town. From 1893 onwards he exhibited at the Salon des Indépendants and after 1895 at the Galerie Durand-Ruel.