After completing his studies, he taught history and philosophy in some high schools, and later became a professor of Institutions of Public Law at the University of Ferrara.

Preti did not hide his socialist ideas and when, in 1941, he was called to arms, he was denounced to the military court for "lese majesty, defeatism and insubordination".

Held in a military prison awaiting trial, he managed to escape the death sentence, thanks to the fall of the regime and the subsequent armistice.

After working in Milan, Preti moved to Switzerland where, in Zurich, he came into contact with Ignazio Silone, who entrusted him with the direction of the periodical "The future of workers".

In 1995 Preti left the PSDI and founded (together with Enrico Ferri) the European Liberal Social Democracy party (SOLE).