Southport 2-1 Grimsby Town
Osayamen Osawe scored a second-half winner to take Southport out of the Conference Premier relegation zone.
Osawe, who had earlier missed a great chance, punished a defensive mix-up between Paul Bignot and Shaun Pearson.
Lenell John-Lewis had put play-off chasing Grimsby ahead in the ninth minute from Ross Hannah's cross.
Scott Brown levelled in the 36th minute from 25 yards before Hannah hit the inside of the post, while at the other end Pearson twice hit the bar.
Grimsby Town manager Paul Hurst told BBC Radio Humberside:
"It's another one where I don't think we got what we deserved, I feel a little bit like Groundhog Day in talking about 'we should have got something, we were good enough'.
"Both goals that we gave away are laughable and that's our own doing. People say it's bad luck, but it's bad play and we were punished on both occasions.
"We need to take advantage of games in hand because if we don't, then clearly we're not in a good position. We need to change it round, we need to force the issue and make it happen."