The 24-year-old didn't contest the cross-country skiing sprint in Italy, won by Bjoergen, and then had to make do with silver behind her Norwegian teammate in the pursuit.
American pair Jessica Diggins and Kikkan Randall took victory in the third women's event, the team sprint, with Bjoergen hotly favoured as individual racing returned for the 10km freestyle.
She set the early pace however Johaug was quicker over the last five kilometres to take her third world title and second individual crown in a time of 25:23.4minutes.
Bjoergen settled for second 10.2seconds behind while Russia's Yulia Tchekaleva edged out biathlon specialist Miriam Gossner for bronze another 22.5 adrift.
With Justyna Kowalczyk not contesting, Johaug trailed Bjoergen by a tenth of a second at the 1.8km mark and, despite losing more time up until 5km, the gap never bridged a second.
By 6.8km Johaug was in the lead, one she wouldn't relinquish, with the deficit almost five seconds before getting up to 11.1 with 600m to go as she skied with teammate Kristin Stoermer Steira.
Johaug relaxed with gold assured over the final metres with Bjoergen settling for second with Tchekaleva third, Gossner fourth and American Elizabeth Stephen fifth.