They were leading 2-0 before allowing Hereford to pull one back and then there were a few hairy moments for the Mariners defence.
Sub Andy Taylor was denied by keeper Wayne Brown when perhaps he should have scored and a goal at that stage would surely have enabled the Mariners to go on to a comfortable victory.
After a poor first half from Hereford's point of view the visitors' 100 per cent away record was in serious danger. They conceded on 20 minutes to a well-worked Grimsby goal.
Full-back Tom Newey not for the first time burst down the left and when his low cross was not cut out by the Bulls defence, midfielder Danny Boshell squeezed his shot in at the far post from close range.
For their first-half dominance Grimsby ought to have had more goals, but they only had to wait until the 52nd minute to double their lead.
They were awarded a free-kick out on the right and Newey's low kick beat everybody including keeper Brown before entering the net at the near post.
After this Grimsby almost let Hereford back into the game with a comedy of errors in defence and although they survived that one Grimsby conceded a goal on 56 minutes.
It was similar in build-up to Grimsby's first goal with veteran striker Trevor Benjamin lashing the ball high into the net at the far post.
After this Hereford became more attack-minded and the game opened up with chances at both ends.
The home side switched to a more attacking 4-4-2 formation as they sought the third goal to put the issue beyond doubt.
This second successive win takes the Mariners on to ten points and a further three points at home to Chester on Tuesday should see them continue their climb of the League Two table.