Pajetta was born in a working-class district of Turin to Carlo, a bank employee, and Elvira Berrini, an elementary school teacher.

He attended Liceo Classico Massimo d'Azeglio for his high school studies[1] and joined the Communist Party of Italy during this time.

In 1927 he was sentenced to two years of imprisonment for subversive propaganda, after having distributed anti-fascist leaflets to the workers at the Saroglia typographical workshops.

[2] In 1933, Pajetta returned to Italy in secret, but was arrested and sentenced to 21 years of imprisonment by the Special Tribunal for the Defense of the State.

In February 1944, together with Ferruccio Parri and Alfredo Pizzoni, he was part of the delegation of the National Liberation Committee (CLN) that sought recognition from the Allies as the legitimate government authority in occupied Italy.