Simenon began working on a local newspaper at age 16, and at 19 he went to Paris determined to be a successful writer. Typing some 80 pages each day, he wrote, between 1923 and 1933, more than 200 books of pulp fiction under 16 different pseudonyms, the sales of which soon made him a millionaire. The first novel to appear under his own name was Pietr-le-Letton (1929; The Strange Case of Peter the Lett), in which he introduced the imperturbable, pipe-smoking Parisian police inspector Jules Maigret to fiction. Simenon went on to write 83 more detective novels featuring Inspector Maigret, as well as 136 psychological novels. His total literary output consisted of about 425 books that were translated into some 50 languages and sold more than 600 million copies worldwide. Many of his works were the basis of feature films or made-for-television movies. In addition to novels, he wrote three autobiographical works—Pedigrée (1948), Quand j'étais vieux (1970; When I Was Old), and Mémoires intimes (1981; Intimate Memoirs), the last after the suicide of his only daughter—and a critically well-received trilogy of novellas about Africa, selections of which were published in English as African Trio (1979).

Despite these other works, Simenon remains inextricably linked with Inspector Maigret, who is one of the best-known characters in detective fiction (detective story). Unlike those fictional detectives who rely on their immense deductive powers or on police procedure, Maigret solves murders using mainly his psychological intuition and a patiently sought, compassionate understanding of the perpetrator's motives and emotional composition. Simenon's central theme is the essential humanity of even the isolated, abnormal individual and the sorrow at the root of the human condition. Employing a style of rigorous simplicity, he evokes a prevailing atmosphere of neurotic tensions with sharp economy.

Simenon, who traveled to more than 30 countries, lived in the United States for more than a decade, starting in 1945; he later lived in France and Switzerland. At the age of 70 he stopped writing novels, though he continued to write nonfiction.