After his release from prison Reimann moved to Ahlen in 1920 to work as a miner, joined the German Coalminer Union and became a full-time official of the Communist Party of Germany (KPD) in 1921.

Throughout the 1920s, he held several positions within the Revolutionäre Gewerkschafts Opposition (RGO), the communist union in the Ruhr area.

After Germany invaded Czechoslovakia in March 1939, Reimann was arrested on 4 April 1939 and imprisoned at Hamm Prison, Sachsenhausen concentration camp and Falkensee.

[2] In 1954, he moved to East Germany but continued to operate his position as the head of the West German KPD, which was banned in 1956.

Reimann returned to West Germany in 1968, became a member of the newly founded German Communist Party in 1971 and its honorary chairman.