Northug produced a fine fightback to finish second in Saturday's race and stay in pole position in the three-stage World Cup, having also come home second in the sprint on Friday.
And by taking victory in Sunday's race in 41.38.3 minutes, coming home ahead of nearest rival Maxim Vylegzhanin of Russia – Northug wrapped up the win in a combined time of 1:05.27.3 hours.
Vylegzhanin's second place on Sunday saw him jump up from 15th place after the first two races to second, 1.4 seconds behind Northug, while Kazakhstan's Alexey Poltoranin dropped down to third.
In the women's race, Marit Bjoergen continued her dominance, putting the finishing touches to a victory that rarely looked in any doubt.
The three-time Olympic champion was in a commanding position, having won both races on Friday and Saturday and on Sunday she didn't let up, coming home in a combined time of 45:35.1 minutes.
Second was Justina Kowalczyk of Poland, who was 1:21.4 minutes behind while Heidi Weng of Norway completed the top three.