Paul Hurst's second spell in charge of Grimsby began with a 2-1 defeat at home to League Two promotion contenders Cambridge.
First-half goals from Adam May and Paul Mullin put the U's in full control at Blundell Park while Elliott Hewitt nodded in a late consolation.
Only four minutes had passed when May scored the opener, courtesy of a deflected strike which left Sam Russell stranded.
It was almost 2-0 soon after but Russell produced a stunning save to deny Harvey Knibbs, who chested down and took aim from close range.
At the other end, veteran Filipe Morais kept Dimitar Mitov honest but Cambridge edged further ahead in the run up to half-time when Mullin broke away and rifled home his 17th of the season.
Grimsby improved upon the restart and twice went close through Kyle Bennett, who first headed and then struck wide either side of the hour mark.
Hewitt pulled one back with 18 minutes to play, courtesy of a looping header, while substitute Montel Gibson hit the post in added time.