Willi Dehmel began to play a role in the activities of the Worker Youth Organization at an early age. Involved initially as head of the Socialist Worker Youth (SAJ) in Hörde and later also of the Proletarian Young Freethinkers, he joined the German Communist Party (KPD) in 1931.

Despite having trained in business and commerce, his political activities meant that after the seizure of power by the Nazis, he could no longer find corresponding employment and so had to get by as an unskilled labourer. After several searches of his home, brought about by his KPD membership, he was arrested, together with his wife, in 1934 and sentenced to 2 years imprisonment for “acts committed in preparation for treason”. Even after completing the prison sentence, he continued to be interned in various concentration camps up to December 1937.

On 31 January 1943, he was called up to serve in Penal Unit 999 (Bewährungseinheit 999) and was transported with the 4th Fortress Infantry Battalion (IV. Festungs-Infanterie-Bataillon) to the Peloponnese in Greece. A resistance group formed within the unit which succeeded in occupying key positions and establishing contact though deserted soldiers to Greek partisans and British liaison officers. The intention was to choose a suitable moment to take the officers and regular soldiers of altogether three “999” battalions by surprise, overpower and disarm them, and flee on board British warships that would be standing by. Willi Dehmel is reported to have been one of the illegal three-man KPD leadership team of his battalion. At the end of May 1944, however, he was arrested along with a number of comrades on the strength of a denunciation. They were charged with “aiding and abetting the enemy and demoralizing the Armed Forces”. All the accused were sentenced to death, and died in the early morning hours of 9 July 1944 under the bullets of a firing squad.