Loiseau was born in Paris and was brought up there, and at Pontoise, by parents who owned a butchers shop. He served an apprenticeship with a decorator who was a friend of the family. In 1887, when a legacy from his grandmother allowed him to concentrate on painting, he enrolled at the École des arts décoratifs where he studied life-drawing. However, a year later he left the school after an argument
While working as a decorator, Loiseau redecorated the apartment of the landscape painter Fernand Quigon (1854-1941). After he left the École des arts décoratifs, he invited Quignon tutor him in painting. In 1890, he went to Pont-Aven in Brittany for the first time, fraternizing with the artists there, especially Paul Gauguin and Émile Bernard. After experimenting with Pointillism, he adopted his own approach to Post-Impressionism, painting landscapes directly from nature. His technique known as en treillis or cross-hatching gave his works a special quality, now recognized as his speciality.