Diethart's debut in the FIS Ski Jumping World Cup took place in January 2011 in Innsbruck. He won his first podium, a third place, in Oberstdorf in 2013. His first world cup win took place just a few days later in Garmisch-Partenkirchen, when he won against his teammate Thomas Morgenstern and the Swiss Simon Amann. After a 5th place in Innsbruck, he proceeded to win his second world cup jump in Bischofshofen, which also meant the triumph of the well-respected Four Hills Tournament.
Diethart competed for Austria at the 2014 Winter Olympics in Sochi, Russia in mens ski jumping in both the individual normal hill and large hill events. In the normal hill he qualified for the finals and placed fourth, while, in the large hill he did not qualify for the finals and placed 32nd.