The three points enables the Millers to leapfrog the Mariners in the league table.
On a blustery afternoon, it was a typically end-of-season game between two poor sides.
But for all their much-publicised troubles, Rotherham showed great spirit and for that alone they deserved to take the points.
Grimsby, remarkably for the home side, did not get a single shot on target throughout the 90 minutes and, although Rotherham did not have many themselves, they did get the one that counted.
The goal on 40 minutes was in keeping with the match. Young defender Ryan Bennett failed to control an awkward long ball and striker Ryan Taylor nicked in to shoot in off both posts.
For the second half Grimsby brought on nippy striker Andy Taylor and midway through the second period introduced fellow frontman Gary Jones, but it had little effect.
Grimsby enjoyed the greater share of the possession, but a tide of yellow shirts behind the ball meant they were never able to get into shooting positions.
The small crowd, boosted by a noisy following from Rotherham, got frustrated with the lack of entertainment as Grimsby slumped to their fourth successive defeat.
For Rotherham, although the future is uncertain, there is clearly no lack of spirit among their players and a good bond between them and the supporters who clearly relished their day out by the seaside.
The fact that it was a poor game did not matter a jot as they celebrated their long-awaited victory.