Born in Saint-Étienne (Loire), Janin's father was a lawyer, and he was educated first at St. Étienne, and then at the lycée Louis-le-Grand in Paris.

[1] Long before, however, he had made a literary reputation for himself, publishing novels such as L'Âne mort et la Femme guillotinée ("The Dead Donkey and the Guillotined Woman") (1829).

[1] Janin authored the text for the song Le Chant des chemins de fer by Hector Berlioz, a composer and fellow critic at the Débats.

[citation needed] After many years of feuilleton writing he collected some of his articles in the work called Histoire de la littérature dramatique en France (1853-1858).

In the early part of his career he had many quarrels, notably one with Felix Pyat (1810-1889), whom he prosecuted successfully for defamation of character.