He led from start to finish at Murcar Links in the fog-shortened 54-hole event, sponsored by the Paul Lawrie Golf Centre.
Law finished with a 71 for a six-under-par total of 207 over one of the toughest courses the event has been played on.
A third-round, three-under-par 68 for 136 put Law four clear of Neil Fenwick (Dunbar) and six ahead of third-placed Greig Hutcheon who had a 69 for 142
Hutcheon, even with a new set of irons which he had regripped only the night before, became the main challenger and got to within two shots of Law after 12 holes of the final round. But that was as close as he got to his Paul Lawrie Golf Centre stablemate.
Law came through a sticky patch to hole some vital putts for par and then birdied three of the last five holes, while Hutcheon could manage only five pars and one birdie for the same stretch.
"That's my fourth win as a pro and it's great to do it in the Northern Open which has to be my favourite tournament," said Law. "Playing on the Challenge Tour has given me a lot of confidence and helped to bring my game up to the level needed to win the Northern Open from this field.
"I'm gaining experience all the time. How good a player could I become? Who knows. I just have to keep improving."