These sides had leaked 12 goals in their first two games so a first half without a goal, or a decent shot for that matter, was a relief to both dressing rooms.
Four minutes into the second half two players who had been given their first starts delivered Rovers' first points. Ryan Green, the hero of Hereford United's promotion, made space for a sizzler of a low cross and Sandell turned it in through 90 degrees.
A win was vital to restore the faith of Rovers fans - the gate was the lowest in the league since they moved to the Memorial Stadium ten years ago, but the result was still overshadowed by Junior Agogo. The big striker is being linked with a move to Nottingham Forest and did everything to stir up this feeling that he will be going shortly.
After taking a knock and being subbed Agogo shook hands with several Rovers players and at the end was shaking the hands of fans too behind the home goal.
His enthusiasm, lacking in earlier games, was back at full blast and his runs provided Grimsby with their main worry in the first half. But when the Mariners were on top they were denied a breakthrough thanks to a fantastic save by Steve Phillips, after Isaiah Rankin had played Michael Reddy through.
Grimsby knew they had to pull back that Sandell goal and quickly veteran Peter Beagrie and young Peter Bore were on to boost the attack.
Beagrie went close with a free-kick lifted over the bar, but 18-year-old Bore, who replaced Rankin, had little chance to repeat his two-goal debut of last week.
Centre-back Ben Futcher came close to saving a point for the Mariners with a late far-post header, but searching as it was Phillips had it well covered.