Seemingly in control of proceedings following a positive start, the Sulphurites conceded twice in the space of five first-half minutes and never really recovered, going on to ship a decisive third before the interval.
Town manager Simon Weaver spoke in the build-up to Tuesday night's game about the need to avoid an "after the Lord Mayor's show" scenario following Saturday's historic FA Cup upset at League One Portsmouth.
And although it cannot be said that his players did not turn up, they lacked sufficient intensity in their defensive work to stop a visiting side full of quality from creating goal-scoring opportunities at will.
Clearly full of confidence after their heroics at Fratton Park - despite lining up with just three recognised defenders in their back-four - the hosts passed the ball tidily for extended spells early on.
A clever bit of play in midfield by Lloyd Kerry saw him slide a pass behind the Rovers back-line for Luke Armstrong, whose low strike from a narrow angle forced Luke McGee into a block at his near post.
That, however, was about as good as it would get for Town in the first half, as they went from bossing possession for more than a quarter-of-an-hour to looking out of the game by the 26th minute.
They fell behind when Forest Green’s excellent right wing-back Kane Wilson beat Nathan Sheron all too easily and was allowed to advance down the right before pulling the ball back for Jack Aitchison, whose strike was deflected past Mark Oxley.
Wilson delivered again from the right flank moments later, hanging up a cross that Matty Stevens looked certain to bury inside the six-yard box, though Sheron managed to clear the ball off the line.
Rovers only had to wait another 60 seconds before their second of the evening did arrive, Stevens running through on goal and taking the ball around Oxley before squaring from the right for Nicky Cadden to apply the simplest of finishes.
Having eventually seemed to have weathered the storm with half-time approaching, Town’s passive back-line was breached once again just before the interval.
Their defending down their left was absolutely criminal, Wilson again given space to weave his way towards the byline, where he slid the ball across the face of goal for Jamille Matt to tap in.
With the majority of the home support presumably wondering why they had bothered braving the cold, they were at least given a brief moment of cheer 11 minutes after the resumption when Harrogate reduced the deficit.
Josh Falkingham played a one-two with Armstrong in centre-field then released Diamond in the left-hand channel.
The on-loan Sunderland winger opted to cut the ball back, his pass eventually reaching Kerry, who shaped to shoot with his right, cut back onto his left and slid the ball into the bottom corner.
The Sulphurites proceeded to make a game of it for a decent spell, Pattison firing well wide before Simon Power drew an instinctive save out of McGee from Diamond’s left-wing cross.
But the match was ended as a contest in the 73rd minute after Warren Burrell made poor contact with the ball as he attempted to cut out a dangerous cross.
Aitchison was then able to dink a centre from the left for Stevens to nod into a gaping net, restoring the visitors’ three-goal cushion and wrapping up the points.
Defeat sees Town drop out of the play-off positions and down two places to ninth.