Grimsby's 18-match unbeaten run came to an end as they were knocked off the top of the Blue Square Bet Premier table with defeat at Braintree.
Luke Daley fired home from the edge of the area after Sean Marks' cross had fallen kindly.
Daley went close to a second soon afterwards but visiting keeper James McKeown beat away his long-range shot.
Craig Disley wasted a good chance to equalise, before Daley scored directly from a corner to seal the three points.
Wrexham's narrow 1-0 victory at Gateshead sent the Welsh side above Grimbsy in the table.
VIEW FROM THE DRESSING ROOM
Grimsby joint-manager Rob Scott told BBC Radio Humberside:
"If you perform like that then you're not going to win games. Tonight we weren't anywhere near good enough to get anything out of the game.
"We could be stood here and made five or six changes and everybody saying 'Why did you make those changes? You still lost the game.' I can't change what's gone.
"We've lost five games this season. I think if we would have said at the start of the season come the tail-end of February that you would have lost five games, then I think everybody would have been snapping your hands off for that.
"It wasn't good enough tonight, the players know that, staff know that and I'm sure the fans know that but we hope it's a blip and we'll certainly be back in on Thursday intent on getting it right for Saturday."