Born in Lublin, Poland, Sadeh began his military career in the Russian army during the First World War. He was decorated for bravery and rose to be a battalion commander. He emigrated to Erez Israel in 1920, upon hearing of the death of Joseph Trumpeldor, whom he had met three years earlier.
There he founded and led the Trumpeldor Labor Battalion (Gedud ha-Avodah). When Arab riots broke out in 1936, Sadeh joined the Haganah. He initiated a policy for defending settlements by going out to attack the marauding Arab bands, rather than remaining behind the barbed wire perimeters of the settlements to await raids. Haganah field companies, which Sadeh commanded, were formed to implement this strategy.
He was also one of the founders of the Palmach, and became its first commanding officer in 1941. In 1945 he was promoted to acting chief of the Haganah general staff, coordinating resistance against the British. During the War of Independence he took part in several operations, including the battle for Jerusalem. He retired from military service at the end of the war.
Sadeh exercised a profound influence on the training, tactics and strategy of the Israel Defense Forces and also wrote articles, short stories, plays and memoirs. He is buried at Kibbutz Givat Brenner.