From 1905 to 1908 he regularly attended the party and trade union training for his knowledge of the principles of social democracy to deepen.
In March / April 1923 Fleißner was appointed by the Prime Minister Erich Zeigner of Police in Leipzig.
Above all, Fleissner it was thanks to them that after a brief, joint custody with Erich Zeigner and Stanislaw Trabalski (November / December 1939) contacts with social-democratic-minded friends like Carlo Mierendorff and Leber to Berlin and to the liberal-conservative opposition circle around him personally known, former mayor of Leipzig Carl Friedrich Goerdeler could be included.
As part of the ongoing Stalinization in the following years, he was on 15 April 1951, as a "hireling of American capital," expelled from the party.
In the conviction that this socialist unity party is doomed, Fleissner died of a heart attack on 22 April 1959 in his Leipzig home.
Mike Schmeitzner: Heinrich Fleißner (1888–1959): Social Democratic continuity of the Empire to the GDR, in: Such pests are also in Leipzig, Frankfurt am Main [u.a.