His parents, farmers living in Colle Val d’Elsa (Siena) after having been manual workers, opened a restaurant, and later a grocery store that went bankrupt.
In 1982 he was one of the curators of the exhibition Anni Trenta: Arte e cultura in Italia, mounted in the Palazzo Reale di Milano (Royal Palace of Milan), the Arengario, and the Galleria del Sagrato.
In 1995, Guerri moved back to Rome and was offered the opportunity to host Italia mia benché, a TV show, with Cinzia Tani, which ran until 1997.
On 1 April 2004, he also took over the direction of L’Indipendente, (where he made himself known for his impartial positions, for criticism of the right, and for having published on the front page an aphorism of the American poet John Giorno: ‘No dick is as hard as life is’).
Guerri defines himself as liberal, libertarian, laissez-faire, and an ex-libertine, like the Partito Radicale, which he has sometimes supported in the past and shares views with, such as the struggle against the death penalty.
The idea behind it was to inform public opinion about the inappropriateness of placing unattractive rubbish bins in areas of cities of high artistic or architectural value.
As cultural advisor, he also promoted the first ‘White Night’ (an all-night arts festival) in Italy, which took place in the Presila region of Calabria on 18 August 1997.
In 2006, he was one of the signers of the manifesto of the Riformatori Liberali, which requested dialogue with the centre-right coalition on various ethical matters like euthanasia, soft drug use, prostitution, and gay marriage.
Appearing at the same time as Renzo De Felice's book on popular acceptance of the Fascist regime, the essay placed him amongst the most authoritative Italian historic ‘revisionists’.
The biography of the party official opened the debate on Fascist culture and its continued influence on Italian society after World War II.
This is a subject to which Guerri returned, contributing to scientific and educational journals (for example, editing the social and political life section of the catalogue for the exhibition Anni Trenta, organized by the city of Milan in 1982).
During this period, Guerri wrote four books that comprise his series on the Fascist ruling class: Rapporto al Duce (Bompiani, 1978), a collection of historic documents; a biography of Galeazzo Ciano, son of the party official as well as Mussolini's son-in-law (Bompiani, 1979); L’Arcitaliano: Vita di Curzio Malaparte, based on previously unpublished documents about his life; and an essay on Italo Balbo.
Il Giornale published a collection of his articles entitled Lezioni di libertà: Contro I pregiudizi, i luoghi comuni, l’omologazione: un grande maestro del pensiero.