In 1938 joined the Irgun, and a year later was sent on a commanders course to Mishmar Hayarden. All the participants were caught by the British during weapons training and sentenced to various terms of imprisonment. Shamir received five years, but was freed two years later.
Held various positions in the Irgun, including deputy commander of Tel Aviv District. When the district commander, Yosef Levi ("Uzi") was killed in the battle for Jaffa, he was appointed commander in his place.
After the establishment of Israel, he was one of the founders of the Herut movement, and served as head of the Organization Division and general director of the movement. Was Betar commissioner and head of the world leadership, and among the founders of the Betar youth village.
Serves as chairman of the Betar youth villages and chairman of the Irgun Soldiers Alliance.