Häßler was a professor at the university of Jena. Under the Nazis, he was member of the "Office for Racial Policy" ("Rassenpolitisches Amt") in Leipzig. In 1939, he co-authored a book on child care, in which he called Jews "rootless parasites" ("wurzelloses Parasitentum").
In 2004, Häßler was one of 22 members of the medical profession to sign a declaration of solidarity for Dr Rosemarie Albrecht, a German doctor accused of having conducted medical experiments in the Nazi era. Häßler was a member of the "Sängerschaft zu St Pauli" in Jena, a student fraternity. He died at the age of 106.