At 14 he joined Betar and later moved on to the Irgun. Of outstanding courage, he soon became a member of the Fighting Force and took part in various operations against the British forces. Among others, he participated in the attack on the British Intelligence offices in Jaffa (December 27, 1945) and the raid on the Kastina military airfield (February 27 1946).
A week later, on March 6, 1946, Yosef set out on another mission, this time in orders to requisition weapons from the Sarafand military camp. In the exchange of fire with the British he was injured, together with his comrade, Michael Ashbel. The two were taken by car in the direction of Tel Aviv in order to receive hospital treatment. The car was forced to halt at a British roadblock where it was searched and the two injured men were discovered and arrested. They were taken to the Jaffa detention center, and after being interrogated, were transferred to a government hospital. Two months later they were tried by a military tribunal, which sentenced them to death, by hanging. They demanded to be recognized as prisoners of war, refused to take part in the court proceedings and chose to make political statements, in which they denied the right of the British to rule Palestine. The kidnapping of five British officers by the Irgun forced the High Commissioner to commute their death sentence to life imprisonment.
Yosef was freed by his comrades in the Acre jailbreak and returned to activity in the Irgun.