Match winner Michael Rippon (95 not out) and Nathan Smith (28) put on a 41-run stand before Hawke's Bay's Blair Tickner (2-58) trapped Smith LBW at 180/6 with just over 20 overs remaining and the run rate in the mid-five's.
But Rippon wouldn't be denied, as after reaching his 50 off 80 balls, his next 45 runs were made off just 31, getting set inside the last ten overs while Dale Phillips (30 from 52) held the line.
Rippon would back away from the stumps to swipe Patel and returning Black Cap Doug Bracewell both to and over the fence, hitting 12 boundaries and two sixes.
Both of his sixes came after Phillips lost his stumps to Bracewell, but by then the pair had put on 84 runs in just over 17 overs, and Rippon knew he could still go for it without risk of defeat, with three and a half overs remaining.
Captain Jacob Duffy took five runs from six deliveries to feed Rippon the strike, and he whacked Bracewell to the boundary to bring up 291/7 with an over to spare.
It was the first time the Volts have outright beaten the Stags in a Plunket Shield game since 2011, and puts a massive hole in their chances of making a title three-peat, with the Wellington Firebirds now 26 points clear on top of the table.
Even if the Stags beat the Firebirds upcoming in Napier, Wellington would still take a six point lead into the last round of the summer, when the Stags will play the Volts again