Omar Bogle's first half penalty was enough for Grimsby Town to claim maximum points in the lunchtime kick-off at One Call Stadium.
Stags created a bucketful of chances in the first half to mark themselves onto the scorecard, but were ultimately unable to beat James McKeown, the Grimsby goalkeeper.
Mansfield boss Adam Murray made two alterations to the starting line-up as Kevan Hurst was drafted into the XI whilst fellow midfielder Oscar Gobern made his home debut after signing last week.
And it was Mansfield who looked the most threatening in the first period, creating their first opportunity on six minutes.
Defensive midfielder Lee Collins did well to feed Danny Rose on the edge of the box, and his curling effort had to be punched away by Grimsby shot-stopper James McKeown.
Then moments later, full back Mal Benning steered a shot wide from outside the Grimsby box.
Stags’ were buoyed by their promising attacking play and Matt Green and Mitch Rose got in the shooting act early on, both having efforts blocked inside the area.
Grimsby’s first attempt came in the tenth minute when a deep free-kick by Ben Davies was headed over by Shaun Pearson.
Stags countered five minutes later, almost to deadly effect, following good link up play between Green and Hurst. And it was the latter who galloped half the length of the field to drift in a cross, which Grimsby had to turn behind.
Then, from the corner, Danny Rose planted a header goalwards, which Ben Davies had to hack off the line.
But it was Grimsby who were to claim the decisive goal from the penalty spot on 18 minutes.
After slipping a ball into the Stags’ box for striker Omar Bogle, the ball ran loose and as Shearer came to claim, he brought down Bogle, leading the referee to point to the spot.
Bogle then stepped up to send Shearer the wrong way and put the Mariners into the lead.
Mansfield tried to reply when Green weaved his way into the box on the left before drawing a save from McKeown, with a rasping drive at the near post.
Stags’ leading marksman latched onto a good midfield pass from Mitchell Rose mid-way through the first half, as home pressure continued. However, Green’s arrow-like shot flew wide of the angle.
Grimsby's Luke Summerfield then looked to drag his low effort well wide, but as it fell inadvertently to Bogle, he failed to divert it into the net.
At the other end, Benning swept a cross in from the left, but Danny Rose was unable to get the direction on his header to test McKeown.
On 41, Danny Rose’s low drive went the wrong side of the post as Stags’ attempted to regain their early momentum.
In time added on at the end of the first period, Alex Iacovitti’s header had to be forced behind for a second successive corner, from which Krystian Pearce headed off target.
Grimsby then entered the second period with the intention of putting men behind the ball, which limited the Stags to fewer chances.
Three minutes after the re-start, Green headed over from six yards after a hanging left-flank cross by Benning.
On 58, Stags made their first change as lively wingman CJ Hamilton took the place of Oscar Gobern.
Then on the stroke of the hour, Grimsby’s Ashley Chambers roamed through the middle and into the area, but his progress was halted by a vital intervention from Iacovitti.
Stags’ boss Adam Murray made a double change as frontman Danny Rose made way for Pat Hoban on 61 minutes and four minutes later, Alex Iacovitti was replaced by Chris Clements.
Scoring opportunities dried up for both sides before Hurst whipped in a good cross for Mitchell Rose, who headed straight at McKeown.
Grimsby created a rare second half attack on 77 minutes, when they broke forward, spearheaded by substitute Shaun Tuton. But his eventual shot from the edge of the area was charged down by Lee Collins to fizz out for a corner.
From the resulting flag kick, Shaun Pearson headed wide.
With 84 minutes played, Green picked out Hamilton near the corner flag but his deep cross was claimed in mid-flight by McKeown.
Two minutes later, Benning delivered into the box with a free-kick as Rhys Bennett headed down, allowing McKeown to smother.
In the dying minutes, Benning crossed from the goal line, but Mitch Rose's forward header was caught by the Grimsby goalkeeper.
Rose had an attempt blocked a minute later as he struck right footed, only for Brandon Comley to throw himself in front of the well-hit drive.
And despite the Stags' late rally, they couldn't find a way through.
STAGS: Shearer, Bennett, Pearce, Iacovitti (Clements 65), Benning, Collins, Gobern (Hamilton 58), M. Rose, Hurst, D. Rose (Hoban 61), Green
UNUSED SUBS: Jensen, Hemmings, McGuire, Henderson
GRIMSBY: McKeown, Davies, Andrew, Pearson, Chambers, Bogle (Vernon 90), Vose, Jackson (Tuton 69), Summerfield, Collins, Comley
UNUSED SUBS: Henderson, Disley, Berett, Bolarina, Boyce
REFEREE: Geoff Eltringham
ATTENDANCE: 4,401 (1535 away fans)