And afterwards he joked: "At this rate I will soon be leading scorer."
His 63rd minute flick from John Oster's right-wing cross proved decisive and FA Cup semi-finalists Watford were left to rue some woeful finishing.
Grimsby's determination saw them through for only their only second victory since Boxing Day but they still face a difficult task to avoid the drop.
Grimsby's scoring opportunities were few and far between but Groves took his chance like a seasoned striker.
Over his career, midfielder Groves has had a good striking record but he had not scored this season before opening his account late last month.
But since then he has popped up with two more vital goals from midfield.
Watford had their chances but some terrible finishing let them down and their winless run in the League now stretches to six matches.
Away from home in the League they have only scored ten goals and that was clearly evident as early as the sixth minute. The recalled Gavin Mahon had a clear chance, but took too long and Grimsby keeper Danny Coyne made a good block.
Grimsby looked unbalanced with just one striker in the side but after the unfortunate Alan Pouton had gone off injured on the half hour there looked a better shape to their side and they started to take the game to Watford.
After the goal Grimsby could have made the game safe with Oster and sub Darren Barnard bringing the best out of keeper Alec Chamberlain.
And it was the veteran Watford keeper who went down bravely at Barnard's feet late on with both players requiring treatment before continuing.