Proud Pompey hit top spot in Nationwide Division One against a struggling Grimsby side who have yet to win this season in five attempts.
But it was a hard-earned three points as they had to battle against a determined Grimsby outfit until their class told in the second half when they finally carved out the decisive opening.
It came seven minutes from time with a classy finish by substitute Mark Burchill, who hit a crisp 15-yard cross-shot that gave Grimsby keeper Danny Coyne no chance.
Portsmouth stepped up a gear after a disappointing first half but keeper Shaka Hislop was so well protected that he had only one save to make and that came in the 17th minute when Grimsby midfielder Stuart Campbell fired in a fine shot but the keeper was well placed to save.
Grimsby played with just a lone striker and it was too much for Crystal Palace's Steve Kabba on his home debut. The on-loan forward lacked support to make much of an impression.
Portsmouth were well backed by over 1,000 travelling fans who enjoyed their day out by the seaside. The late winner should make the long journey back to the south coast a lot more bearable.
Portsmouth boss Harry Redknapp admitted afterwards that his side had made a slow start and it took them a while to get going but he was pleased with the second-half showing which maintained his side's bright start to the season