The 22-year-old Norwegian star emerged from the ski jumping stage in top spot with 134.8 points, ahead of older brother Harald Johnas who began 32 seconds adrift at the start of the cross-country course.
But while it was something of a family affair on the hill, Harald Johnas was unable to maintain his good form and duly dropped down the rankings, while Jarl Magnus cruised to the win, 44.3 seconds ahead of runner-up Vinzenz Geiger of Germany.
Geiger was one of six athletes to enter the stadium together at the 2.5km point but did enough to win the tactical battle to finish 0.2 seconds ahead of Norway's Joergen Graabak in third.
Klingenthal
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Following victory, Jarl Magnus Riiber said: "I'm so humbled to have got the win and I'll do my best to continue that form going forward into the rest of the competition."
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