​​​Ossietzky was the publisher of the magazine Die Weltbühne and was sentenced, in 1931, to 18 months in prison for revealing military secrets.

​The main reason for his indictment was that the magazine drew attention to the Reichswehr's armament, which was prohibited by the Treaty of Versailles, and disclosed documents related to this practice.

​He was released on 22 December 1932, after serving 227 days of his sentence, because of an amnesty for political prisoners.

​On 30 January 1933, Adolf Hitler was appointed Reich Chancellor and the NSDAP's seizured of power in Germany. Because of the decree on the state of emergency issued after the Reichstag fire and the suspension of civil liberties on 28 February 1933, Carl von Ossietzky was arrested that same night and imprisoned at Spandau prison. He would later be transferred to the Esterwegen concentration camp and then to others.

​During his time at prison and the camps he would be subjected to physical violence, interrogation and deprived of food. He remained in concentration camps and prisons until 1936.

​Ossietzky's arrest was met with widespread responses abroad. In Norway, a group of writers (including Nini Roll Anker, Johan Borgen, and Olav Duun) published a protest petition in Aftenposten on 14 December 1935. They also called for the prisoner to be awarded the Nobel Prize, which Ossietzky ultimately received for 1935.

​Ossietzky following his release in 1936 would die in May of that year. He had contracted tuberculosis whilst in a concentration camp and would die of the disease. Even whilst he was being treated in hospital, he was under the surveillance of the gestapo. He died at Nordend Hospital, Berlin-Pankow.

​A memorial commemorates and remembers Ossietzky in the Pakow region of Berlin.