As a student at the Politecnico di Torino, he became active in the youth wing of the Italian Socialist Party (PSI), and engaged in political propaganda from a Marxist perspective.

Longo was a fervent anti-fascist, and, when Benito Mussolini established his Fascist regime in Italy in 1922, he emigrated to France where he became one of the principal leaders of the PCI.

He would return to Moscow several times in the years to come, with a specific expertise in political ideology, and was to meet Joseph Stalin and other members of the Soviet Union leadership.

Longo took part in the Spanish Civil War as an inspector of the Republican troops in the International Brigades under the leadership of Randolfo Pacciardi, and took the nom de guerre Gallo.

It was at Dongo on Lake Como on 28 April 1945 and whilst being escorted by the Garibaldi Brigade partisans that Mussolini and his mistress Claretta Petacci were executed; the extent to which Longo took part in the killings has been the subject of dispute by historians.

[2] He was subsequently elected, and repeatedly re-elected, to the Italian Chamber of Deputies on the PCI list and was a member of the party leadership.

In late 1968 Longo suffered a stroke; although he partially recovered in the subsequent months, from February 1969 he was assisted in most decisions by Enrico Berlinguer acting as vice-secretary.