Number three batsman Stacy Hyndman was main saviour for Northland as he hit 55 from 42 balls as Chris Page, Mark Andrianatos and Fletcher Coutts provided key scores of 20 or more.
All out for 141 off the penultimate ball of the innings, Northland would have been nervous having not posted above 200 for the first time in the competition.
However, any nerves were settled as left-arm tweaker David Armitt took two early wickets to leave Counties Manukau at 28 for 2 after 6.3 overs, before the rain came and the game was eventually abandoned with Northland ahead by 14 runs on the Duckworth-Lewis-Stern method.
Northland player/coach Neal Parlane commended Hyndman on a fantastic knock and Armitt on what Parlane considered his best ever bowling spell.
"[Armitt's] plan was so very simple and clear, and he executed it perfectly, Parlane said.
"[Counties Manukau] didn't have a clue against him so it was nice to see a young guy like that do well."
The trophy's namesake, Brian Dunning, was a legend of Northland cricket and represented Northern Districts for about 20 years. Parlane said it was great to bring the trophy home after a disappointing last-place finish in 2018.
"I certainly had a goal of winning this tournament and we regard this tournament as ours, so it's always nice to bring the trophy back to what we think is its home."