He fought in the Franco-Prussian War and after the war became a journalist, writing art and theater criticism as well as political articles for various newspapers in Paris. In 1885 Mirbeau was converted to anarchism, and remained a firm believer in that philosophy until his death. Continuously writing articles for anarchist periodicals, he was well-known as one of the leading anarchist men of letters.
He also became a very important art critic, especially in the 1890s, writing at length on the Impressionist and avant-garde movements in Montmartre and Montparnasse, believing it to be the foundation of a cultural revolution in France. He was a founding member of the Académie Goncourt.
Octave Mirbeau is credited with the discovery of the Belgian writer, and Nobel Prize winner, Maurice Maeterlinck, and he contributed significantly to the careers of Camille Pissarro, Auguste Rodin, Claude Monet, and other important artists of the time.
Responsible for more than 1,200 short stories and articles, Mirbeau was also an author of plays and full length novels. His two best-known novels are Le Jardin des supplices (The Torture Garden) and Le Journal d un femme de chambre (The Diary of a Chambermaid). Mirbeau suffered from depression all his life, and his plays are primarily dark comedies.
Despite his leftist views, Octave Mirbeau was an astute financier who accumulated a small fortune. He died on his 69th birthday on February 16, 1917 and was interred in Cimetière de Passy, Paris, France.