Wiessner started climbing with his father in the Austrian Alps before World War I. At the age of 12, he climbed the Zugspitze, the highest peak in Germany. In the 1920s, he established hard climbing routes in Saxony and the Dolomites that have a present-day difficulty rating of up to 5.11. This was at a time when the hardest free climbing grade in the United States was 5.7. At the age of 25, he made the first ascent of the Fleischbank in Tyrol, which was proclaimed the hardest rock climb done at that time.