Seibt completed training as a metal spinner in 1922–1926 and worked until 1933 as a civil engineering worker and a stone setter.

From 1934 to 1939, Seibt was a stagehand and a theatre master at the Deutsches Theater Berlin, and also a member of the outlawed KPD's party leadership.

In 1939 he was arrested and in 1941, he was sentenced by the Volksgerichtshof to life at hard labour in a Zuchthaus for "undermining German fighting forces and conspiracy to commit high treason".

From 1952 to 1964, he was first secretary of the SED Potsdam district leadership,[1] although this was interrupted by his studies at the CPSU Party College in Moscow in 1956–1957.

[2] Seibt received the Patriotic Order of Merit (Vaterländischer Verdienstorden) in 1965, the Karl-Marx-Orden in 1968 and 1988, the Clip of Honour for the Fatherland Order of Merit in 1973, the Star of the Friendship of Peoples (Stern der Völkerfreundschaft) in 1978, and the Great Star of the Friendship of Peoples (Großer Stern der Völkerfreundschaft) in 1983.