[1] Valiani was born Leó Weiczen in Fiume (now Rijeka), on the Adriatic Sea (then in the Hungary part of Austria-Hungary, now in Croatia), to a Hungarian Jewish family.

[5] In 1939, after the defeat by the Nationalist faction of Francisco Franco, he fled to France where he was detained as a political prisoner in Camp Vernet together with Arthur Koestler, who wrote about it in his book Scum of the Earth, before later fleeing to Mexico.

In 1943, the British Special Operations Executive sent Valiani secretly behind enemy lines in Italy across the unstable front between the Allied and Axis forces to Rome.

He moved northward to work with Italian resistance movement leader Ferruccio Parri and with Milan's anti-fascist National Liberation Committee.

As a leader of the resistance to fascism in the north, Valiani helped organise the final partisan uprising in April 1945, and put his signature to the document ordering the execution of the captured fascist dictator Benito Mussolini.