In atrocious weather conditions, the Stags took the lead on the half-hour mark when Lindon Meikle’s drive was deflected into the net via Eddie Oshodi.
Matt Green almost doubled the advantage on the stroke of half-time when his looping header from James Jennings’ set-piece hit the woodwork.
But the visitors eventually added a second less than two minutes after the re-start when Lee Stevenson slotted beyond Sam Russell after Green pounced upon an error by Chris Stokes.
Nevertheless, Forest Green fought back bravely in the second period and could have reduced arrears when Oliver Norwood’s drive rattled the woodwork close to the hour mark.
Aarran Racine wasted a gilt-edge in the dying stages, before the hosts eventually halved the deficit three minutes from time as Norwood tucked away from close range.
But Mansfield held on in pouring rain to record a magnificent victory, which moves them three points clear at the top and heaps pressure on their title-rivals.
Manager Paul Cox named an unchanged side for the clash, and it was Mansfield, wearing their change strip of white shirts and blue shorts, who got proceedings underway.
The Stags began on the front foot and Adam Chapman tested the resolve of Forest Green’s back-four with two long-throws deep into the six-yard box in the early exchanges.
Nevertheless, the hosts wasted the first real chance of the game, as Wes Burns fired high and wide in the sixth minute following Kieron Forbes’ intricate through ball.
Forbes, who scored in this fixture last term, disappointingly scuffed an effort into the arms of Alan Marriott on the quarter-hour mark, after weaving his way through the middle with a swashbuckling run.
Both teams had begun to foil each other, whilst heavy rain that poured continuously from the skies above New Lawn Stadium, made playing conditions extremely difficult.
By the midway point in the first half, neither side were in full control, though Mansfield were at least enjoying superior possession than their play-off chasing counterparts.
The Stags then had to make a substitution, as Louis Briscoe replaced Junior Daniel, who limped off following an earlier collision with Rovers right-back Jared Hodgkiss.
However, on the stroke of the half-hour mark, Lindon Meikle broke the deadlock in fortuitous circumstances.
The winger capitalised on a defensive mistake by Oshodi, before unleashing a low drive that looked to be going yards wide until Oshodi inadvertently diverted it into his own net.
With a lead established, the Stags gradually began to dominate the tight midfield battle that had preceded the opener, which frustrated their Gloucestershire hosts.
The Stags, however, picked up the first caution as Lee Beevers went into the referee’s notebook for a tackle on Forest Green playmaker Paul Green.
Burns then wasted one of few chances that Rovers created in the first period, when he woefully high and wide, following an exquisite one-two with Norwood on the left flank.
But it was Mansfield who still looked the brighter, and substitute Briscoe almost added a second when his 35-yard snap-shot forced a back-peddling Russell into a superb save.
From the resulting set-piece, Luke Jokes towered above everyone else as he met Jennings’ superb delivery, but the centre-back was unable to find the target.
Jennings again caused problems for the Rovers rear-guard as Mansfield almost doubled their lead in stoppage-time at the end of the first period.
Russell flapped wildly and made no connection to Jennings’ fine free-kick towards the back-post, where Green instead made a connection and headed onto the crossbar.
Nevertheless, just two minutes after the re-start, Mansfield again ruthlessly punished more defensive mistakes by the hosts in order to add what proved to be a crucial second goal.
Having dallied on the ball, Stokes was dispossessed by Green, which allowed the striker to feed an inviting ball between two defenders towards a galloping Stevenson.
Oshodi hurridly back-tracked to try and get close to Stevenson, but the midfielder retained his composure and slid the ball underneath an on-rushing Russell to double the lead.
Rovers boss Dave Hockaday cut a forlorn figure in the touchline; the lashing rain and silent atmosphere around the stadium perfectly summing up the abject misery felt by those connected to Forest Green.
With their slim play-off chances dwindling with each passing second, Hockaday was forced into a substitution as Matt Taylor replaced the ineffective Yemi Odubade.
Forest Green desperately needed a goal to arrest their ailing fortunes and it so nearly arrived when Norwood’s swerving 30-yard drive rattled the woodwork with Marriott rooted to the spot.
Rovers grew with confidence and threw on Jan Klukowski in favour of Al Bangura on the hour mark.
The Stags were being forced back, but the central-defensive partnership of Jones and John Dempster remained resolute as they dealt with the pressure that was being thrown at them.
Nevertheless, the hosts did manage to create gilt-edge openings; none more so than when Taylor’s shot across the face of goal almost resulted in Norwood sliding home.
Burns also tormented Mansfield with his trickery, but the on-loan Bristol City striker failed to test Marriott with a left-footed effort he poked wide on 71 minutes.
Mansfield continued to soak up pressure and battering-ram substitute Matt Rhead, who had earlier replaced Green, soon added his presence to defensive duties.
Following Norwood’s clever lay-off to Oshodi on the edge of the area, Rhead’s crucial block prevented Marriott from being called into what would have been a low save.
The hosts spurned another golden opportunity to reduce arrears in the closing minutes, when Racine woefully blazed over at the back-post following Taylor’s intricate flick.
But just as Mansfield were preparing themselves for a comfortable finale, Green bundled his way through and squared a simple pass to Norwood, who slotted beyond Marriott.
With a grandstand finale now set-up, Forest Green poured forward as the game entered five minutes of added time, knowing they needed an equaliser to keep alive their play-off hopes.
But every ball they hurled forward; every creative flick or trick attempted by Racine, Norwood or Burns, was foiled by a resolute Mansfield defence who refused to be breached for a second time.
After enduring five gruelling minutes of pressure, the full-time whistle was met by joyous celebration from the travelling supporters, who knew their side had just earned what may prove to be a pivotal win.