Deaglio was born in Turin, where he graduated in medicine (June 1971) and worked in the Mauriziano Hospital.

In the mid-1970s, he started his journalist career for the communist newspaper Lotta continua, of which he was director from 1977 to 1982. Later he worked for numerous national newspapers and magazine, such as La Stampa, Il Manifesto, Panorama, Epoca and l'Unità and was director of Reporter in 1985-1986.

From the later 1980s he worked for the TV journalist show Mixer, devoting in particular to Sicilian mafia and to foreign events. In the 1990s he hosted several journalist shows in Rai Tre, such as Milano, Italia (January-June 1994), Ragazzi del '99 (1999), Così va il mondo, Vento del Nord and L'Elmo di Scipio. From 1997 to 2008 he was director of the weekly Diario.

In 2006 his documentary Uccidete la democrazia!, supposing cheats in the electronic vote for the 2006 Italian general elections in favour of Silvio Berlusconi's party, House of Freedoms, raised controversies. Later his hypothesis was discarded by an official recount of the schedes by the Italian Parliament.