Answer by taking a quote from the following article:

Erich Mielke was born in a tenement in Berlin-Wedding, Kingdom of Prussia, German Empire, on 28 December 1907. During the First World War, the neighborhood was known as "Red Wedding" due to many residents' Marxist militancy. In a handwritten biography written for the Soviet secret police, Mielke described his father as "a poor, uneducated woodworker," and said that his mother died in 1911. Both were, he said, members of the Social Democratic Party of Germany (SPD).

Mielke was then put on trial for ordering the shootings of East Germans who were trying to defect to the West. In November 1994, the presiding judge closed the proceedings, ruling that Mielke was not mentally fit to stand trial.  During his incarceration, at JVA Moabit corrections officers supplied Mielke with a red telephone like the one in his office at Stasi Headquarters. Although it was not connected to the outside world, Mielke enjoyed having imaginary conversations with non-existent Stasi agents. His other favorite pastime was watching game shows on television.  In 1995, parole officers and Mielke's attorneys argued that he was "totally confused" and obtained his release. At 87 years of age, Erich Mielke was Germany's oldest prison inmate and had been incarcerated for 1,904 days. Days before his release, the Public Prosecutor of Berlin announced that he was "not interested in chasing an 87 year old man anymore" and that all further prosecution of Mielke had been indefinitely suspended.  According to Koehler:  [Mielke's] bank account, which held more than 300,000 marks (about US $187,500), was confiscated. Before his arrest in 1989, the most feared man in East Germany had lived in a luxurious home with access to an indoor pool. In addition, he owned a palatial hunting villa, complete with a movie theater, trophy room, 60 servants, and a 15,000 acre hunting preserve. After he was released from prison Mielke was obliged to move into a two room, 600-square-foot flat. Like all Stasi pensioners, he would henceforth have to live on 802 marks (about US$512) a month.

could you give me more information about his imprisonment please?
During his incarceration, at JVA Moabit corrections officers supplied Mielke with a red telephone like the one in his office at Stasi Headquarters.