Born in Milan, Mieli debuted as journalist at 18 for L'Espresso, where he remained for some 20 years. As a member of Potere Operaio he initially adhered to far-left positions. Later he took a more moderate stance under the influence and tutelage of his teachers, Rosario Romeo and Renzo De Felice.
From the 1980s Mieli worked for the most important Italian newspapers. After one year and a half at La Repubblica, he was hired by La Stampa in 1987. He became director in 1990. Two years later he moved to Il Corriere della Sera during the Tangentopoli bribe scandal. In May 1997 he was replaced by Ferruccio De Bortoli, assuming the position of editor-in-chief of RCS MediaGroup, publisher of Corriere della Sera. He continued his collaboration for that newspaper and returned as its director on December 24, 2004.
- Horowitz, Jason. World Briefing | Europe: Italy: Storm Over New Broadcast Chief. New York Times, March 13, 2003. Retrieved September 15, 2010.
- Catholic News Agency. Director of Italian newspaper joins in defense of Pope Pius XII. October 13, 2008. Retrieved September 15, 2010.
- BBC News. TV row strains Italian government. March 13, 2003. Retrieved September 15, 2010.