Town wrapped up their pre-season schedule with a defeat as they were beaten by Chesterfield at the Technique Stadium.
Unusually for a summer friendly, the match gave the Mariners an opportunity to test themselves against opponents from the same division, with the National League kick-off still a week away.
And it was the hosts who made the better start in Derbyshire as Liam Mandeville opened the scoring from their first attack, before doubling Chesterfield's advantage on 21 minutes.
But after Erico Sousa halved the deficit, Town had the better of the latter part of the first half as they looked to get back on level terms.
Both sides had chances after the break but the tempo dropped as several changes were made by each management team and despite Seb Revan hitting the bar and Sousa drawing a good save from home keeper Scott Loach, the Mariners were unable to find an equaliser.
Will Bapaga and Alex Hunt went straight into the Town starting line-up after becoming the club's newest recruits when they joined on loan from Coventry City and Sheffield Wednesday respectively this week.
James McKeown started in goal, with Michee Efete, Shaun Pearson, Luke Waterfall and Adam Crookes forming the defence. Bapaga, Giles Coke, Hunt and Revan began in midfield, with Sousa and John McAtee lining up in attack.
The Grimsby substitutes were Max Crocombe, Ryan Sears, David Longe-King, Ben Fox, Luke Spokes and Luis Adlard.
Former Mariners striker Stefan Payne came up against his old club in the colours of Chesterfield after making the move away from Blundell Park earlier in the summer.
The home side took the lead within a minute of the kick-off, with their first attack. MANDEVILLE got the ball midway inside the Town half and took it forward before firing an unstoppable drive past McKeown and into the top corner of the net from 25 yards.
Efete headed over the bar from a Hunt free-kick, before a Sousa cross was cut out before it could reach Revan as the visitors looked to respond.
Town won the first corner on eight minutes but Hunt's delivery from the right beat everybody and went out for a throw-in.
Efete conceded a corner at the other end soon afterwards and it turned into another one when Jamie Grimes' header was turned behind by Waterfall. This time, the referee spotted a foul as the ball came across and blew for a Town free-kick.
A foul by Hunt gave Chesterfield a free kick on the left on 17 minutes but it was headed clear by Crookes. The home side kept possession and a return ball was also dealt with, this time by Pearson.
An ambitious effort by Jeff King was blocked, then a good challenge by Efete stopped a cross from the home left, at the expense of a corner.
Efete stayed down and had to receive treatment and when the flag-kick was eventually taken with the right-back off the pitch, it led to Chesterfield doubling their lead. The ball from the corner was not fully dealt with by the Town defence and when it came back to taker Calvin Miller, he kept the move going and a defensive mix-up was capitalised on by MANDEVILLE, who poked past McKeown for his second goal of the game.
Efete was passed fit to play on but Chesterfield were continuing to look the more dangerous of the two sides and King fizzed a shot over the bar from the right, while at the other end, the Mariners were yet to test home keeper Scott Loach.
But the visitors were suddenly back in the contest on 33 minutes as they grabbed a goal against the run of play. SOUSA was the man on the scoresheet as his 20-yard shot took a heavy deflection which wrong-footed Loach and spun into the net.
And there was almost an equaliser soon afterwards when a free-kick from the left was headed back across goal by Waterfall, although Pearson's header at the far post was deflected away for a corner.
The Mariners' revival continued with another corner, this time on the opposite right flank, but Waterfall's goalbound header from it was blocked and the ball was eventually gathered by Loach.
Crookes missed the target after working an opening for a shot from the edge of the box, then Town's next attack saw McAtee win a corner on the right. The ball from it was played short and Hunt made his way to the fringe of the penalty area before being stopped by Miller.
That proved to be the last opportunity of the half for either side and after a poor start saw them go two goals down, the Mariners had grown into the game and shown much more positive signs after Sousa made it 2-1.
Crocombe and Longe-King replaced McKeown and Pearson before the restart, with Jack Clarke coming on in place of Saidou Khan for the hosts.
Danny Rowe hammered an early free-kick into the Town wall, before Mandeville missed an opportunity to complete his hat-trick.
At the other end, McAtee crafted an opening for a shot but his low drive from a narrow-angle on the right was well-blocked for a corner by keeper Loach. From the flag-kick, the ball eventually broke for Revan, whose dipping effort from 25 yards clipped the top of the crossbar.
Payne left the field to boos from the travelling Town supporters on 54 minutes, to be replaced by Jak McCourt.
A cross from McCourt on the right was comfortably caught by Crocombe, then a foul by Revan on King gave the hosts a free kick on the right. The ball from it was headed behind for a corner, which was cleared by Waterfall.
Town were given a chance of their own when a loose ball in the home midfield was picked up by Sousa. He advanced before letting fly from 30 yards but Loach was equal to the effort, tipping it over the bar for a corner.
Another opportunity came from the flag-kick as the ball broke for Waterfall to hit a shot on the turn, although it was beaten away by the keeper.
A triple-substitution by Chesterfield came on 68 minutes and saw George Carline, Alex Whittle and a trialist introduced in place of Miller, Fraser Kerr and King.
A home free-kick four minutes later was headed behind for a corner, which eventually led to Laurence Maguire lifting a shot over the bar from 20 yards.
Coke and Efete made way for Fox and Sears soon afterward, before more home substitutions saw Nathan Tyson and Jim Kellermann sent into the action in place of Mandeville and Rowe.
Crocombe got down to stop a low shot by Tyson on 83 minutes as the home side looked for a third goal which would make sure of a victory.
Spokes replaced Bapaga for the last seven minutes of the game and Town continued to attack but they were unable to find a way through as Chesterfield saw out the remainder of the contest to seal a 2-1 win.
All thoughts now turn to next Saturday's opening National League fixtures, with the Mariners set to begin their season at Bromley.
Town: McKeown, Efete, Waterfall, Pearson, Crookes, Revan, Coke (c), Hunt, Bapaga, Sousa, McAtee.
Subs: Crocombe, Sears, Longe-King, Fox, Spokes, Adlard.