Johannes Firn competed in his first FIS competition in 2005. On January 8, 2007, he made his debut in the Nordic Combined B World Cup in Klingenthal when he reached 36th place in a sprint. A little more than two years later, on January 25, 2009, with eighth place in Kranj, Slovenia , he was able to achieve a place in the points for the first time in a competition as part of this B-World Cup, renamed the Continental Cup in 2008 . At the Nordic Junior World Ski Championships in 2009 in Štrbské Pleso , he finished 14th in the individual.
Johannes Firn completed his last international competition on March 16, 2014 as part of the Continental Cup in Kuusamo, Finland . He lives in Frauenwald .