Émile Alfred Marie Dezaunay was born at 3 rue Du Guesclin in Nantes on 25 February 1854, into a prosperous merchant family. His father died in 1857 and he was raised by his mother, Caroline Tronson, a first cousin of Jules Verne, under the guardianship of his uncles.

The Nantes painter Jules-Élie Delaunay recommended him for the École des Beaux-Arts in Paris, and he entered Delaunay's studio in 1875. He also studied under Pierre Puvis de Chavannes. He began exhibiting at the Salon of the Société des Artistes Français in 1878 and married in Nantes in 1884.

The turning point came in 1886, at the Triennial Exhibition of Fine Arts in Nantes, where he met Maxime Maufra. The friendship lasted the rest of their lives; the two toured Brittany together, sometimes by bicycle, sometimes by train, painting side by side. In 1890 Maufra brought him to Pont-Aven, where he lodged at Marie-Jeanne Gloanec's pension and met Gauguin, Chamaillard and Jourdan, painting two summers running at Le Pouldu. When Maufra took a studio at the Bateau-Lavoir in Montmartre in 1892, Dezaunay joined the circle of Nantais who gathered there with Aristide Briand and the poet Victor-Émile Michelet. That same year he showed at Le Barc de Boutteville's second exhibition of Impressionist and Symbolist painters, alongside Bernard, Denis, Filiger, Maufra and Sérusier.

Married with a family, he was never tempted by bohemia. His oils stayed loyal to a gentle Impressionism; the Pont-Aven lesson surfaces instead in his etchings and aquatints, printed in Paris with Auguste and Eugène Delâtre. He helped found the Salon d'Automne in 1904 and exhibited at the Exposition Universelle in 1900. His most productive years ran from about 1892 to 1909.

He settled permanently in Nantes in 1913 and died there on 4 June 1938.