Avni was born December 25, 1924, in Jerusalem, as Yosef Danoch. After the establishment of the State of Israel, he changed his name to Avni (his underground cover name). Avni went to work as a locksmith at a young age, to help provide for his family (learned his job at his cousins' workshop, the Sabah brothers).
At 16 Avni joined Betar and four years later (in 1942) joined the Irgun.
In the second attack on the British Intelligence offices in the Russian Compound (December 27, 1945) Avni was in charge of the sappers unit. He also took part in the attack on the King David Hotel and the Schneller Army Camp.
Avni was arrested in 1947 and detained by the British in Latrun Camp. Avni was released in March 1948 and took part in the conquest of Deir Yassin, where he was wounded attacking the Muchtar's house.
Sources: The Irgun Site