Leading from the ninth minute, they were pegged back by a Mansfield equaliser 11 minutes from time.
This was the least the Stags deserved and it was rough justice when Grimsby grabbed a winner four minutes from time.
Impressive Grimsby striker Michael Reddy scored Grimsby's first and had a big hand in the late winner.
Winger Andy Parkinson set up Reddy and he did well to score from an acute angle past veteran keeper Kevin Pressman.
It was Mansfield who had started the brighter and midfielder Stephen Dawson blazed over early on.
Neither sides' keeper was seriously tested during the first half but Grimsby will have been more then happy to reach half-time a goal to the good.
Mansfield were almost in complete control after the change round and Dawson was twice denied by fine stops from Grimsby keeper Steve Mildenhall.
It seemed that Mansfield would score sooner rather than later but it took a penalty to break the deadlock.
When a ball came into the box from the right, sub Giles Coke was fouled by Grimsby midfielder Paul Bolland. There was a two-minute delay while both players received treatment, but the break did not faze Mansfield skipper Richie Barker who coolly slotted the penalty away.
At this point Mansfield looked a good bet for all three points, although Grimsby did up their game and had the ball in the net following a corner but the flag was clearly up for an infringement.
Then with four minutes remaining, Grimsby full-back Tom Newey set off on a surging run from his own half and he played a short pass to Reddy. The shot was blocked by Pressman, but it fell invitingly to Parkinson in the box and he gleefully tapped home.
Grimsby now set off on three away trips in eight days but, as they have won eight times on their travels and only lost once, it will be a trio of matches they will probably relish.