Carlisle boss Keith Curle made a double change at half-time and they found a way back into the contest when Jamie Devitt scored a penalty after Richie Bennett had been fouled.
Bennett and Sam Cosgrove both missed great chances to level the scores in quick succession and striker Hemmings then scored his second goal of the game to seal the win.
The hosts were straight on the attack from kick-off, winning an early corner as they looked to set the early tempo. Lee Angol’s header from the set-piece did not seriously test United keeper Jack Bonham, though, the tempo for the first half had been set.
With five minutes on the clock another Mansfield corner, this time from the right, saw Alex MacDonald head wide an Alfie Potter delivery.
But two minutes later away captain Danny Grainger delivered from the left and found Tom Miller inside the penalty area, only for the full-back to steer wide of Conrad Logan’s left-hand post from an excellent position with undoubtedly the Blues’ best chance of the half.
And the Stags took full advantage of that miss and took the lead moments later. Cumbrian Grainger tried to find Bonham, although his back-pass was cut out by Hemmings who rounded Bonham and slotted home.
As United looked to hit back, Shaun Miller, part of the new-look frontline alongside surprise inclusion Cosgrove, and former Stags man Reggie Lambe were looking particularly threatening.
Man-of-the-match Hemmings then burst down the left on 20 minutes as he looked to turn provider, although Brentford loanee Bonham did just enough to thwart Angol.
At the other end, a short corner routine between Devitt and Bermudian Lambe presented Grainger with a shooting opportunity. But the Carlisle captain fired miles over Mansfield goalkeeper Logan’s crossbar on 25 minutes.
Two minutes later, a poor pass out of defence by Jacob Mellis presented Mike Jones with an opening, around 20 yards from goal. However, the former Oldham man – set to play against one of his former clubs in Sheffield Wednesday in the FA Cup on Saturday – also couldn’t hit the target.
Evans’ pre-season title favourites then doubled their advantage on the half-hour mark. A mistake from Tom Miller allowed the Stags to cross. Bonham did superbly to keep out Angol’s initial header, although defender Rhys Bennett was on hand to slot home his second of the season from close-range.
Four minutes before the break, Potter again drove down the right and crossed for Hemmings, with the former Rangers striker powering his header narrowly over.
Ahead of the match, United manager Curle had said he was looking forward to taking on Mansfield manager Evans and his no.2 Paul Raynor. But it was certainly the Stags pair who were the happier of the two managerial camps at half-time, with ex-Stags boss Curle engaging in a long conversation with referee Darren Drysdale as the sides headed off the pitch.
It came as little surprise that Curle opted to make a double substitution at half-time. Hallam Hope and Richie Bennett coming on for Grainger and Shaun Miller. Those changes also saw a switch in formation with Tom Parkes moving to left-back and Hope joining the action on the left wing, while Irish midfielder Devitt moved into a more central position.
The changes certainly made an impact for United, with joint top scorer Hope looking particularly threatening on the left.
But the Cumbrians still were yet to create a clear-cut chance since Tom Miller’s early miss as Lambe peppered Logan’s gloves on the 55th minute.
With both sides struggling to create opportunities after the restart and the Stags looking comfortable, CJ Hamilton could not beat Bonham on 65 minutes.
Moments later, Carlisle were handed a route back into the match. The hosts’ goalkeeper, Logan, was adjudged to have fouled sub Bennett by referee Mr Drysdale. Devitt slotted home from the spot to cut the deficit in half.
As Mansfield looked to restore their two-goal cushion, Mellis fired over from just outside the box.
A minute later, United had two golden opportunities to level in the rain. Firstly, Cosgrove glanced a Hope left-wing cross against Logan’s left-hand post before Bennett hit the bar with the rebound from close-range.
Not for the first time in the game, the Stags cashed in on a bad Carlisle miss as Hemmings steered home Potter’s delivery to make it 3-1 to give Mansfield’s play-off bid another boost and United’s another significant dent.
John O’Sullivan was the next player Curle threw into the fray as Lambe came off against his old club to a chorus of boos.
Hemmings could have claimed his hat-trick five minutes from time, though, his shot was blocked by Bonham.
But, this time, United could not force their way back into the contest.
Midfield duo Kelvin Etuhu and Luke Joyce were not involved in the Blues squad, with the latter instead sent to scout Carlisle’s FA Cup opponents Sheffield Wednesday in action in their 3-0 defeat to Burton.
Full-back James Brown was not used in the final match of his current half-season loan.
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