In his youth, Basso worked on Critica sociale, Il Caffè, Avanti!, Coscientia, Quarto Stato, and Pietre - which he directed in 1928, initially from Genoa, then from Milan.

In April 1928, Basso was arrested by the Fascist authorities in Milan and interned on the island of Ponza, where he studied for his degree in philosophy.

In 1934 he once more took up politics as director of the Centro Interno Socialista, with Rodolfo Morandi, Lucio Luzzatto and Eugenio Colorni.

After 25 July (when Benito Mussolini was ousted by a coup d'état inside his Grand Council of Fascism), the movement joined with the PSI to form the Italian Socialist Party of Proletarian Unity (PSIUP), with Basso as one of the leading figures.

Post-1945, Lelio Basso was elected Vice-secretary of the PSIUP, and, in 1946, became a deputy to the Italian Constituent Assembly which consecrated the Republic.

In December 1963, he made a voting statement to the Chamber, signed by 24 members of the minority of the Socialist parliamentary group against the first Center-left government (led by Aldo Moro).

Lelio Basso's life was a medley of intellectual activity and research on the one hand and the search for an effective political instrument on the other, all on an international scale.