Reimann was born in Elbing (Elbląg), West Prussia (today Poland). He worked as a riveter at the Schichau yards in 1912–16 and was drafted into the German Army in the First World War. In 1913, he became a member of the German Metal Workers' Union and the Socialist Labourers Youth, in 1916 of the Spartakusbund.
In 1918 he was sentenced to 1 year imprisonment for his participation in an anti-war demonstration at Elbing throughout the German Revolution of 1918–19. 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. Reimann fought against the French Occupation of the Ruhr in 1923 and was imprisoned for a short time. Throughout the 1920s, he held several positions within the Revolutionäre Gewerkschafts Opposition (RGO), the Communist union in the Ruhr area.
After the Nazis took over power in Germany in 1933 Reimann continued his work, now in illegal underground and became the head of the RGO in 1934. In 1935 Reimann was a delegate at the 7th Congress of the Comintern in Moscow and later worked for the KPD-Foreign office in Prague. After Germany invaded Czechoslovakia in March 1939 Reimann was arrested on 4 April 1939 and imprisoned at Hamm Prison, Sachsenhausen concentration camp and Falkensee.
After the end of World War II, Reimann was a candidate of the Western KPD organization for the executive committee of the Sozialistische Einheitspartei Deutschlands (SED) but had to quit as the SED activities were limited to East Germany. In 1948, Reimann became chairman of the West German Communist Party.
He was the head of the KPD group at the parliamentary council and a member of the Bundestag in 1949–53. In 1954, he moved to East Germany but continued to operate his position as the head of the West German KPD, which was illegalized in 1956.
Reimann returned to West Germany in 1968, became a member of the newfounded German Communist Party in 1971 and its honorary chairman.
Reimann died in Düsseldorf.