In a dominant display, the Norwegian led from the off and quickly opened up a gap from the chasing pack, eventually prevailing in 36:54.5 on the Seefeld snow.
'Staggering!' - Therese Johaug dominates to win 15km Skiathlon
It was enough for a comfortable 57.6s advantage over compatriot Ingvild Flugstad Oestberg, who held off Natalia Nepryaeva in a dramatic battle for silver, the Russian finishing just 1.1s behind the Norwegian.
In-form Pellegrino kicks off New Year with sprint success
The result comes after Johaug missed the 2017 Worlds whilst serving the doping ban that also ruled her out of the PyeongChang Winter Olympics, with the 30-year-old looking to add to her tally as the Championships continue with the team sprint event on Sunday.
Kilde completes Val Gardena double with Downhill victory
Petra Vlhova leads the way after first run