Northug had already been in action three times ahead of Wednesday's 15km freestyle race, picking up silver in the sprint event, finishing an agonishing fourth in the 30km pursuit and sixth with Norway in the team event.
But despite never having won a 15km event at a World Championships or even World Cup before, Northug was clearly a man possessed as he finally topped the Italian podium.
The two-time Olympic champion came home in 34:37.1 minutes for his first gold of the Championships, his eighth career world title.
Finishing 11.8 seconds further back in second was Sweden's Johan Olsson while Norwegian Tord Asle Gjerdale settled for bronze – their first medals of the Championships.
With two more podium finishes Norway extended their lead atop the overall medal table, sitting pretty on 14 with Northug's success their fifth gold.