Koplenig carried out work in support of the Bolsheviks at the time of the October Revolution in 1917, and was a member of a Workers' and Soldiers' Council in Nizhny Novgorod from 1918 until 1920, when he returned to Austria.

When the KPÖ was banned by the regime of Engelbert Dollfuss in 1934, Koplenig went underground and continued to work illegally in Austria for a while before fleeing the country.

He remained in exile after the Anschluss of Austria into Nazi Germany and the outbreak of World War II, and spent time in Czechoslovakia, France, and the Soviet Union.

Koplenig returned to Austria after the end of the war in 1945, and was made Vice-Chancellor in Karl Renner's provisional government, together with Adolf Schärf and Leopold Figl.

[3] A statue of Koplenig is placed in front of the building of Globus-Verlag, the KPÖ's publishing house, in Brigittenau, Vienna.