After placing third in the Four Hills opener in Oberstdorf, the 21-year-old gave warning of his intent by taking the win at the second leg in Garmisch-Partenkirchen.
That was Diethart's first-ever World Cup victory and propelled him into a lead for the Four Hills title that he would not let go at either Innsbruck or Bischofshofen.
A fifth in Innsbruck hardly reduced his overall lead, he still led by almost ten points, and he wasn't fazed putting in a stunning performance to claim the title on home soil.
The Austrian leapt 138.5m for first place during round one in Bischofshofen and then 140m during round two to maintain top spot and claim the Four Hills crown.
And at 21 years and 315 days old, Diethart is the youngster winner of the Four Hills since Janne Ahonen's victory at the age of 21 years and 240 days old in 1999.
He won in Bischofshofen with a points total of 296.5 while his overall score was 1012.6 with fellow Austrian Thomas Morgenstern progressing to second.
Peter Prevc, who finished fourth overall, split the pair on the day in Bischofshofen with Morgernstern third with 293.6 to the Slovenian's total of 294.8.
Simon Ammann, fourth in the final round, had to make do with third overall as his bid to win all five major ski jumping titles was forced to wait for another year at least.
Only Matti Nykanen has won the World Cup, World Championship, including ski flying, Olympic and Four Hills titles with Ammann starting so well with victory in Oberstdorf.
It wasn't to be for him though as he finished third overall with 992.4 points, Prevc fourth with 971.3 before Noriaki Kasai of Japan in fifth with 962.1.