Gaboriau first achieved publishing success with L’Affaire Lerouge, serialised in 1865, which featured the amateur detective, Tabaret, who recurs in his later novels.

In the Petit Journal of the same date, Timothée Trim feigned ignorance and astonishment, asking "What can this Monsieur Lecoq be?"

On 15 May Millaud finally revealed to the public that Monsieur Lecoq was the title of a long work by Emile Gaboriau that they were going to publish.

[4] On 24 May newspaper vendors asked for a considerable increase in copies for the day Monsieur Lecoq was published for the first time.

Binyon observes that this is a common structural characteristic of Gaboriau's novels, which separates the different subjects of each part: "Each novel falls into two-halves: the first begins with the discovery of the crime and narrates the activities of the detective; the second, which usually takes the form of a complicated family history, describes the events leading up to the crime.

[10] As Gaboriau admired Poe, it is not surprising that Lecoq and Dupin share many traits,[11] and Murch observes, "they both regard a mysterious puzzle as a challenge to their powers of perception; they reason with mathematical precision and enjoy giving little lectures on 'algebraic analysis' to their associates.

"[12] L'enquête Policemen on patrol in a dangerous area of Paris hear a cry coming from the Poivrière bar and go to investigate.

Gévrol, judging from the man's attire, concludes that he was a soldier, and the name and number of his regiment are written on the buttons of his great coat.

He finds footprints in the snow outside the back exit to the bar, revealing the presence of two women, who were helped to escape by an accomplice.

Lecoq continues his investigations the next day, following leads on the two women, but when he goes to report to M. d’Escorval he discovers that he has broken his leg and will be replaced by M. Segmuller.

After making further enquiries, including observing the prisoner from above his cell, fail to produce any information, Lecoq decides to take drastic measures.

Mai wanders in the streets, followed by Lecoq and Absinthe in disguise, and eventually comes out of a seedy bar with a suspicious-looking man.

In the evening, they stop outside a town house, which belongs to the Duke of Sairmeuse and Mai scales the wall, eluding his followers.

Stopping at nothing that could help him succeed, Lacheneur is even welcoming to Martial, the marquis de Sairmeuse, who is enamoured with Marie-Anne and hopes to make her his mistress.

His fiancé, Blanche, the daughter of the marquis of Courtomieu, is furious and vows revenge on the woman she wrongly considers as her rival.

Chupin's eldest son dies, Blanche believes that she is free from the blackmail, but Jean Lacheneur arrives in Paris, aware of who killed his daughter, and decides to exact vengeance on her by using her husband.

He makes Chupin's widow begin the blackmail again, and sends an anonymous letter to the duke to draw attention to her movements.

Martial is stunned when he sees the seedy bar that his wife has been going to, but glimpses the truth when he finds out that it is owned by Chupin's widow.

Jean Lacheneur has set a trap, in which he intends to lead Martial and Blanche to a notorious place and provoke a scene in which they will find themselves compromised.

Having found out that Blanche has committed suicide and that M. d’Escorval has been reunited with his son, Lecoq decides to confront the Duke of Sairmeuse, having put together all the pieces of the mystery.

One day a red-haired man goes to the Duke's house and gives him an urgent letter from M. d’Escorval, asking him, as a gesture of his gratitude for not revealing his identity, to lend him a large sum of money that he needs.

[17] Goulet highlights the prominence of logical reasoning in Gaboriau, which is a feature of the writing of most detective fiction authors.

[19] He also states that Gaboriau, with Lecoq, 'introduces detailed visual scrutiny to the genre,’ eliciting an account of past events from inanimate objects.

[22] Conan Doyle wrote, 'Gaboriau had rather attracted me by the neat dovetailing of his plots, and Poe’s masterful detective, M. Dupin, had from boyhood been one of my heroes.

But could I bring an addition of my own?’[23] Conan Doyle also uses Gaboriau's two-part structure for two of the four longer Sherlock Holmes stories.