After taking four points from their last two games against League Two's top two side, Town handed Crawley Town their first win in six after going down 3-2 at the Checkatrade stadium.
Goals from James Collins and Jordan Roberts (2) – the second straight from a corner – consigned Town to their first league defeat under Marcus Bignot.
Omar Bogle – the only shining light on a gloomy afternoon for Town – did add two more to his tally, but it wasn't enough, despite a late rally.
Not a man who's afraid to shake things up, Town's boss went with two changes here, with Craig Disley and Dominic Vose coming in for starts.
And it was the latter, with a curling effort that flew wide, who fired the first warning almost straight from kick-off with Crawley's Adi Yussuf squandering a six-yard header as both teams made bright starts.
The hosts, however, arguably enjoyed the better chances of the opening exchanges, with James Collins crashing a sharp effort into the side netting after less-than convincing defending from their visitors.
However, a sharp switch of play from Zak Mills put Town back on the front foot and Kayden Jackson cut inside and brought the game's first save from Glenn Morris with a stinging low effort from the edge of the area.
As the fans sang his name with their trademark ditty in his honour, Bogle lifted over from just inside, five minutes later, with Luke Summerfield blasting over after some pinball in Crawley's area.
That passage of play rather summed up the opening 20 minutes of a scrappy affair littered with mistakes, but entertaining nonetheless.
James Collins swept a sharp effort at McKeown from 16 yards, but when Town broke from their keeper's parry, Bogle only managed to withstand Josh Yorwerth's tugging back for so long, and the Crawley centre-half became the first to go into the book.
Lewis Young followed him shortly after for a lunge on Jackson that left the Barnsley loanee in a heap, with Danny Collins heading over from the resulting free-kick.
However, it was the Town defender's opposing namesake who opened the scoring on 31 minutes and it all came about from the Mariners' own attack.
Bogle was denied at one end and the clearance was hoisted up to forward to Yussuf. He edged towards the edge of the box before sweeping a pass out to James Collins on the far side. The frontman's sharp check-out sent Gowling sliding, leaving him to tuck the ball past McKeown for the opener.
They wouldn't hold their lead until half-time, however.
Having been quiet for most of the opening 45, Vose picked up a nice area of space and set Bolarinwa free in the right channel. The winger's cross took a handy deflection that allowed Bogle to beat his man and nod past Morris for the leveller and his 15th of the season.
With no changes at the break, the pattern of the second half looked set to be very much the same. Crawley started brightly but saw an early chance snuffed out while, buoyed by his goal, Bogle cut in from the left and fired in a dipping effort onto the roof of the net from 25 yards.
However, four minutes short of the hour mark and Town were hit with another counter-attack, and another goal.
Crawley swept forward and a long ball found Collins free on the right. He pulled back to the edge of the box, where Roberts expertly steered past McKeown with the outside of his boot to regain the lead.
Despite having three midfielders centrally, it was too often proving an area of weakness for Crawley to exploit. Bignot planned changes, but not before his side twice cleared off the line following a corner.
First, McKeown produced a fine save to deny the excellent James Collins from yards out before Town's own Collins deny the same man from the follow up.
Brandon Comley and Rhys Browne replaced Disley and Jackson respectively, but that didn't stem the tide, with Yussuf firing over when one-on-one with McKeown after yet another break.
Time was running out for Town and unfortunately for the travelling fans, it didn't look like getting any better for their side.
In fact, it could have got worse earlier than it did with Collins pouncing on Mills' error to drift into the box and fire into the legs of McKeown.
But the disappointing Vose then passed the ball straight out for a corner that Town had to defend with 10 men while Collins was off to clean up a bloodied nose.
Roberts spied a gap at Town's front post and curled the set-piece straight in, leaving McKeown and Bogle to argue over whose fault it was, but it would matter little.
Bogle did thump home a superb goal from 20 yards to add to his ever-increasing tally, but despite his best efforts – and those of BOlarinwa who drew an excellent save from the keeper in the final minute of five added on – Town had lost.
Grimsby Town: McKeown, Mills, Collins, Gowling (c), Andrew, Bolarinwa, Summerfield, Vose, Disley, Bogle, Jackson.
Subs: Pearson, Chambers, Browne, Vernon, Jones, Comley, Henderson (GK).