Derby picked up their first away win when two goals from midfielder Adam Bolder cancelled out an early strike from Grimsby's Darren Barnard making his full debut.
Derby answered manager John Gregory's call for a more competitive away display but they face much tougher challenges if they are to regain their Premiership status at the first time of asking.
Gritty Grimsby felt they deserved a point and they had a good case on an afternoon which must have seen a new record established for the length of a half when the first period lasted an amazing 60 minutes following a serious injury to Grimsby striker Steve Livingstone.
Livingstone was taken to hospital for a brain scan after falling awkwardly following a strong challenge by Derby defender Danny Higginbotham.
The word was that Livingstone was ok although he had drifted in and out of consciousness.
Grimsby who had started in lively fashion went ahead when midfielder Barnard making his full debut lobbed the ball into the empty net after keeper Mart Poom had headed clear.
But Derby equalised before half time when Bolder headed in an Adam Murray cross and the same player was on hand after 74 minutes to knock the ball into the roof of the net after a shot had been blocked on the line.
Grimsby hit back through sub Phil Jevons and his header appeared to have snatched a late point until he was ruled to have handled the ball into the net.
Derby picked up their first away win back in Division One while Grimsby with just one point from their opening three games know they face a massive task if they are to survive after twice cheating relegation in the last two seasons