Graduated in Judaic Studies at the Hebrew University of Jerusalem and was part of the Sohba (Comradeship) - a group of students whom later played key roles in the Irgun, including David Raziel, Avraham Stern and others.
At the end of 1937, Kook was sent to Poland and engaged in organizing illegal immigration. When World War Two broke out, he went to the United States together with several other commanders (including Aryeh Ben Eliezer and Alexander Raphaeli). At first they promoted the establishment of Jewish armed units within the allied armed forces to take part in the war effort against Nazi Germany . When information came through about the mass extermination of Jews, the group set up the Committee for the Rescue of European Jewry.
When the Revolt was proclaimed (1944), Hillel Kook, together with several comrades, established the Hebrew Committee for National Liberation. The Committee appealed to the American public and its elected representatives, and won considerable support for the Irgun's activities in Eretz Israel.
When Israel was established, Koook returned home and was elected to the First Knesset on behalf of the Herut movement.