Plenel spent his childhood in Martinique and his youth in Algiers, Algeria.

He briefly worked for Le Matin de Paris in 1980, before moving to the French newspaper Le Monde, where he worked as the paper's education editor (1980–82), legal columnist (1982–90), a reporter (1991), head of the legal department (1992–94), chief editor (1994–95), assistant editorial director (1995–96), editor (1996–2000), and editor-in-chief (2000–04).

[1] From 1985 to 1986, while working for Le Monde, he was one of the targets of a wiretapping scandal perpetrated by a secret presidential anti-terrorism cell, which he had implicated in the "Irish of Vincennes" affair for framing three Irish nationals on terrorism charges.

[3] He resigned from the editorial staff of Le Monde in November 2004,[4] and left the newspaper on 31 October 2005.

In 2023, Plenel accused the actress and filmmaker Maïwenn of assaulting him in a Parisian restaurant, alleging she grabbed him by the hair before spitting in his face.