Having been put into bat, Otago was dismissed for 237 on a good pitch at Cobham Oval in Whangarei.
It was perhaps 20 or so runs short of a good total and the home side reached its target with three wickets and three deliveries to spare.
The win enables Northern Districts to shore up its position in second place on the competition table with 17 points from six matches.
Competition leader Central Districts lost to Wellington but stays on top with 18 points.
Wellington moves equal with Otago on 12 points, Canterbury is on 11 points, with Auckland trailing the field on eight points.
Last Monday, Otago lost to Auckland by three wickets when Paul Hitchcock swung the last delivery of the match away for six to win the game.
Despite consecutive losses, the defending champion is still within striking distance and returns to Dunedin to play Canterbury on Wednesday and Auckland on Saturday.
Having won the toss, Northern Districts quickly prised out former Black Caps opener Aaron Redmond when he was bowled by Graeme Aldridge for seven.
Fellow opener Shaun Haig smashed a quick-fire 50 from 54 balls and combined with Greg Todd for a valuable 61-run stand for the second wicket.
It was Haig's second one-day 50.
The aggressive right hander has been in good touch and has scored 196 runs at an average of 49 this season.
At 85 for one, Otago looked well placed, but all-rounder Joseph Yovich got Todd when he offered Trent Boult a catch and then nicked out Otago skipper Craig Cumming from the next delivery.
Yovich claimed his third scalp moments later when Haig made a mistake and was caught by Boult.
With the innings teetering, Derek de Boorder (43) and Nathan McCullum (39) added 57 for the sixth wicket and Ian Butler hit one six and two fours in a brief but entertaining innings of 23.
Martin was the pick of the bowlers with two for 30 from 10 overs and former Otago seamer Bradley Scott took two for 43 from nine overs.
Northern's reply got off to a dreadful start, with BJ Watling making a two-ball duck.
That brought Michael Parlane to the wicket and the veteran looked in commanding form, blasting 44 runs from 53 balls before he was bowled by James McMillan..
The home side wrested control of the match with James Marshall and Kane Williamson combining for a 69-run stand for the fourth-wicket.
But Butler, who finished with three for 34 from 10 overs, struck, removing Marshall. McCullum then got a delivery to hold up and caught and bowled Williamson.
With two new batsmen at the crease, Otago brought strike bowler Neil Wagner back into the attack.
Another wicket and the tail order would have be exposed.
However, former Otago keeper Peter McGlashan and Yovich put on the best partnership of the game to secure the win.
The pair added 77, plundering 17 runs off Wagner in the 43rd over.
But, with victory in sight, McGlashan got a top edge and was caught on 51 and Yovich was bowled by Butler for 27.
After losing its first four matches, Auckland has now won two on the trot after chasing down Canterbury's 223 for seven at QEII Park Village Green yesterday, NZPA reports.
Michael Papps scored a patient century for the host but it was not enough as Auckland reached 226 for seven with 11 balls remaining.
Wellington turned the State Shield form book on its head with a six-wicket defeat of competition leader Central Districts in Wellington yesterday.
Coming off two losses, Wellington cruised to Central's challening 263 for six in the 47th over, with Chris Nevin's boundary seeing it home at 267 for four.
