Born in Viroflay, Yvelines, he is the brother of scientist Axel Kahn, and son of a Jewish father and a Catholic mother. Having obtained a degree in history, he started work at a postal sorting office, then at a printing works. He soon moved into journalism and was sent to cover the war in Algeria, undertaking the journalistic investigation that became known as the ‘Ben Barka affair’. Kahn then worked as a reporter for Paris-Press, L'Express and Europe 1. He later moved to Le Monde as special correspondent for North Africa. In 1977, he became editor of the compilation of the ‘Nouvelles Littéraires’ and in 1983, was named editor of Matin de Paris. In 1984, he created L'Événement du Jeudi then in 1997, together with Maurice Szafran, started the weekly magazine, Marianne, where he was the editor in chief until 2007. He often writes under the pseudonym of François Darras or Serge Maury.
Kahn has taken a clear position on many media subjects, including :
Denouncing economical liberalism in 1995
Denouncing the intervention of NATO in Serbia in 1999
Denouncing the American intervention in Iraq in 2003
He adopted a positive position on the European Constitution in 2005 but denounced the failure of the press to provide a proper platform for those who opposed it
In 2007, he actively supported UDF candidate François Bayrou for the presidency