Grimsby wrapped up their first league double of the season - courtesy of Portsmouth centre-back Andy Awford.
The veteran defender stuck out a foot as Tony Gallimore's first cross-shot flew into the box from the left.
It was debatable whether it would have hit the target, but that was immaterial as Awford steered the ball past goalkeeper Andy Petterson.
It was rough justice on a Pompey side who dictated much of the play at a high tempo.
Steve Claridge went closest to grabbing a point as he turned neatly on the edge of the box and fired in a snap shot which Danny Coyne tipped around the post at full stretch.
Portsmouth laid siege to the Grimsby goal towards the end, and substitute Guy Whittingham saw his point-blank shot deflected over the bar by Gallimore.
Soon after, Sammy Igoe's free kick was well held by Coyne.
Grimsby were able to attack on the break as Pompey pushed men forward, and the Mariners' best chance fell to Alan Pouton.
He raced on to a through ball and had Petterson beaten with a low cross-shot which went inches outside the post.
Earlier, Bradley Allen stung Petterson's hands with a vicious low shot after being set up by Jack Lester in the 11th minute.
Grimsby appealed for a penalty when Lester went down under challenge from Awford, but referee Hill was not interested.
Portsmouth, defending in depth and breaking at speed, almost caught out Grimsby in the 44th minute.
Fitzroy Simpson robbed Wayne Burnett and raced to the byline before crossing the ball in the direction of Claridge.
The striker was about to nod home when full-back Danny Butterfield appeared from nowhere to head clear.
Simpson himself went close with a 25-yard free-kick which flashed just past the far post.
Butterfield also came to Grimsby's rescue when charging down a powerful shot from Matthew Robinson.