In January 1933 the political context changed dramatically when the Nazi Party took power and converted Germany into a one-party dictatorship. In the aftermath of the Reichstag fire which took place at the end of February, the new government took the opportunity to arrest large numbers of people, including former Reichstag members who had represented left-wing parties. Mache was arrested in March 1933 and detained till December 1933. He was held at Dürrgoy and Lichtenburg concentration camps. He was re-arrested in August 1944, during the course of a mass round up of people with records of left-wing political activity, which came in the wake of a failed assassination attempt against the leader, and taken to Gross-Rosen concentration camp. Here he died a couple of months later. According to the testimony of Paul Löbe, who was detained in the same concentration camp at the same time, the immediate cause of Karl Mache's death was a heart attack.