Ricky Ravenhill was in the thick of the first-half action, with the County midfielder testing Nick Colgan with two volleys and then sending a third inches wide. But he blotted his copy book with a yellow card for a two-footed tackle on Peter Sweeney and was fortunate not to be sent off after hauling back Wes Fletcher.
Grimsby's Tommy Wright headed over from a Sweeney corner and then the striker was booked for a two-footed lunge on Neal Bishop,
The crucial goal came from a long clearance from goalkeeper Kasper Schmeichel which was missed by centre-back Paul Linwood. Hughes raced clear and hit a right-foot shot into the far corner.
Grimsby went close to an equaliser when an overhead kick from substitute Nathan Jarman was clawed away by Schmeichel.