Lemonnier was born in Ixelles, Brussels. He was a member of the symbolist Jeune Belgique group, but his best known works are realist. His first work was Salon de Bruxelles (1863), a collection of art criticism. His best known novel is Le Mâle (1881).
He studied law, and then took a clerkship in a government office, which he resigned after three years. Lemonnier inherited Flemish blood from both parents, and with it the animal force and pictorial energy of the Flemish temperament. He published a Salon de Bruxelles in 1863, and again in 1866. His early friendships were chiefly with artists; and he wrote art criticisms with recognized discernment. Taking a house in the hills near Namur, he devoted himself to sport, and developed the intimate sympathy with nature which informs his best work. Nos Flamands (1869) and Croquis d'automne (1870) date from this time. Paris-Berlin (1870), a pamphlet pleading the cause of France, and full of the author's horror of war, had a great success.
His capacity as a novelist, in the fresh, humorous description of peasant life, was revealed in Un Coin de village (1879). In Un Mâle (1881) he achieved a different kind of success. It deals with the arnours of a poacher and a farmer's daughter, with the forest as a background. Cachaprès, the poacher, seems the very embodiment of the wild life around him. The rejection of Un Mâle by the judges for the quinquennial prize of literature in 1883 made Lemonnier the centre of a school, inaugurated at a banquet given in his honour on May 27 1883. Le Mon (1882), which describes the remorse of two peasants for a murder they have committed, is a masterpiece in its vivid representation of terror. It was remodelled as a tragedy in five acts (Paris, 1899) by its author. Ceux de la glèbe (1889), dedicated to the "children of the soil," was written in 1885.
He turned aside from local subjects for some time to produce a series of psychological novels, books of art criticism, etc., of considerable value, but assimilating more closely to French contemporary literature. The most striking of his later novels are:
Lemonnier spent much time in Paris, and was one of the early contributors to the Mercure de France. He began to write at a time when Belgian letters lacked style; and with much toil, and some initial extravagances, he created a medium for the expression of his ideas. He explained something of the process in a preface contributed to Gustave Abel's Labeur de la prose (1902). His prose is magnificent and sonorous, but abounds in neologisms and strange metaphors.