His older brother, Kurt Sindermann, also entered politics as a member of the Communist Party and sat on the Saxon state parliament from 1929 to 1933.

The group was banned by the Nazi regime and in June 1933, Sindermann was arrested and condemned to eight months of imprisonment for illegal political activities.

In March 1935, he again was arrested for attempted high treason, tortured and put in solitary confinement for six years at Waldheim jail.

In 1941, he was transferred to "protective custody" to several concentration camps, first at Sachsenhausen, then at Mauthausen in Upper Austria, and finally at Ebensee, until being freed by the arriving U.S. army in 1945.

After 1946 he was a member of the Socialist Unity Party (SED), created in April 1946 from the forced merger of Communists and Social Democrats in the Soviet Occupation Zone.

As a representative of the Socialist Unity Party, Horst Sindermann spoke during the commemorations of the liberation of the Sachsenhausen concentration camp at the National Memorial of the GDR.