Grimsby were kept at bay by a battling performance from 10-man Portsmouth at Blundell Park.
The visitors had midfielder Russell Perrett sent of in the 14th minute but the Mariners could not subdue Alan Ball's' side.
Perrett went for an off-the-ball incident involving Grimsby striker Jack Lester. Referee Kevin Lynch consulted his assistant before showing the red card.
Portsmouth did not sit back, however, and John Aloisi raced clear to fire home his 17th goal of the season in the 23rd minute.
Aloisi beat three Grimsby defenders for pace and then clipped the ball over the advancing Andy Love.
Grimsby hit back three minutes later with Paul Groves' eighth of the season. The Grimsby skipper was set up by Lester and prodded the ball over keeper Andy Petterson and into the top corner.
Petterson, on loan from Charlton and making his debut, denied Lee Nogan with a fine one-handed save five minutes before the break.
And the impressive keeper stood between Grimsby and victory in the second-half.
Substitute Kingsley Black looked certain to score in the 71st minute after being set up by Nogan but Petterson dived to his right to save. Two minutes later the keeper kept out a close-range volley from Lester.
Lester then had another effort cleared off the line as Portsmouth held on.