Lady Luck also smiled on Cambridge a second time a minute from the end as a Luke Waterfall equaliser was ruled out for offside.
Cambridge now go on to face Luton Town at home in round three with new Stags boss Adam Murray left to reflect on what might have been or should have been, given the number of times a lively Stags managed to get the ball into the United box.
After the thrills of the 2-2 draw at the Abbey Stadium, the excitement continued with an end-to-end game that could have ended up 6-6.
There was little quality on show but lots of opportunities were wasted as both teams made mistakes.
Mansfield’s defending was very nervous and could have proved more costly in the end.
But, going forward, with Chris Clements pulling the central midfield strings and Rob Taylor showing how easily he can go past players, Stags always looked a threat at the other end on a horrible, cold, wet night that drew less than 2,000 fans.
This is the second year in a row that Stags have drawn an FA Cup second round away tie only to lose the home replay.
Murray is still searching for his first win since being appointed manager full time and was angry over his side’s first 45 minutes display.
Had Vadaine Oliver taken a great chance after just 35 seconds, Stags might have enjoyed a very different evening.
But it wasn’t to be with maybe the highlight for the home side the cameo display of 17-year-old full debutant Dan Fletcher for the final nine minutes.
It was all-change again as Murray went 3-4-3, his third different formation since taking over.
He was forced into three changes with loanees Keiron Freeman, Junior Brown and Dimitar Evtimov not permitted to play.
Also left out were Reggie Lambe and Fergus Bell, both dropped to the bench.
In came Sascha Studer, Rakish Bingham, Lee Beevers, Rob Taylor and Ollie Palmer, Bingham and Palmer flanking Oliver up front with Beevers and Taylor wide in a midfield four.
Oliver might have given Stags a dream start when Bingham curled in a great cross from the left after just 35 seconds.
But Oliver rose from just five yards out to plant his header straight at Dunn.
On four minutes Chadwick beat Studer to the ball in the box but had to take it wide to the by-line and Tafazolli headed away his eventual cross with the keeper back in position.
That forced the first of two away corners, but Studer was untroubled by either, Champion having a shot blocked from the first.
A great through ball from Heslop was only just too hard for the chasing Bingham as Dunn got there first.
But disaster struck for Mansfield on 10 minutes as United took the lead through a stroke of fortune.
Kaikai’s shot from the edge of the home box took a massive deflection off Waterfall as he tried to close him down and that left Studer wrong-footed and unable to re-adjust himself, the ball trickling over the line with the keeper helpless to get to it.
Two minutes later Oliver almost levelled matters, Palmer headed a long Taylor cross back across goal where Oliver then tried to head the ball inside the far post but saw it drift the wrong side of the upright.
On 18 minutes Taylor fed the ball to his Bingham to his right, but his low shot lacked venom and was easily held by Dunn.
Beevers completely missed a ball he should have cut out which saw Appiah race forwards, the Stags man happy to see Appiah’s low cross miss both team mates who had ran up with him in support,
Tafazolli had to go off the field for treatment to a head injury and came back wearing a bandage.
Stags withstood another flurry of Cambridge pressure then Bingham wanted too long at the other end when a shooting chance presented itself and Taylor was able to block for a corner.
Oliver was then wide with a low, hopeful shot from 25 yards as home frustration grew.
Sutton was booked for a foul on Blair on 29 minutes once play had stopped, the referee playing advantage as Blair leapt over Sutton’s lunge.
Rob Taylor drew jeers from the away fans for a poor finish well wide as we reached the half-hour mark.
Clements’ cutting pass put Palmer into the box only for Miller to slide in and superbly block.
Then a mistake by Greg Taylor allowed Palmer to speed away into the box, but his low cross was cut out.
Heslop was caught in possession on 38 minutes after holding onto the ball far too long, suddenly United had a man over in the box and worked a series of passes to set up Blair for a low shot that Studer blocked with his legs.
On 42 minutes Palmer headed on a long Beevers free kick, but Oliver just couldn’t make contact in the box.
Studer had to palm away a corner, then fist away Blair’s ball back into the six yard box.
Clements almost levelled in stoppage time as a long Beevers ball in was cleared to him just outside the box. He kept his first time blast down and on target, but it took a deflection and went for a corner.
The cold weather took a turn for the worst at the break as rain began to fall to greet the returning sides.
Mansfield threatened first as Palmer headed over from a great Taylor cross from the left.
Waterfall allowed Kaikai to dispossess him and then get goalside on 51 minutes, his subsequent holding of the Cambridge man earning him a booking.
Mores sloppy defending by Stags saw Tafazolli give the ball to Appiah in the box, but then he redeemed himself with a block to concede a corner.
Lambe took over from Palmer down the right on 55 minutes.
Dunn punched away a Clements corner two minutes later, then Lambe had to get back well as Cambridge threatened a breakaway.
A loose pass from Waterfall allowed sub Bird to run forward and let fly from 20 yards on the hour, Studer beating the shot away.
On 67 minutes Lambe was narrowly wide of the post after controlling a Clements pass and getting a difficult shot away.
Two minutes later Rhead came on to replace Oliver in the home attack.
On 78 minutes Beevers became the third home player booked after a foul.
Teenager Fletcher came on for his debut with nine minutes to go, replacing Beevers, and caused problems with his first touch in the box as his attempted shot was quickly closed down.
Bingham had a shot deflect wide of the post from a Fletcher cross as Stags won their fifth corner of the half.
Rhead was too high with a far post header from a long Sutton cross as the home side threw everything at Cambridge.
With one minutes to go a Mansfield corner was cleared, but when it was put back into the box, a scramble ensued from which Waterfall found the net with an overhead kick, his joy short-lived as an offside flag ruled his effort out.
Fletcher forced another corner in the four minutes of stoppage time from which Waterfall headed over as time finally ran out on Mansfield’s hopes of a cup run.
MANSFIELD TOWN: Studer; Sutton, Tafazolli, Waterfall; Beevers, Heslop, Clements, R. Taylor; Bingham, Oliver (Rhead 69), Palmer (Lambe 55). Subs not used: Riley, Bell, McGuire, Bishop.
CAMBRIDGE: Dunn, Tait, G. Taylor, Coulson, Miller, Champion, Hughes, Appiah, Chadwick (Naylor 74), Blair (Bird 58), Kaikai (B.J. Taylor 67). Subs not used: Norris, Simpson, Nelson, Atkinson.
REFEREE: Carl Boyeson of East Yorkshire.
ATTENDANCE: 1,920 (305 away). CHAD STAGS MAN OF THE MATCH: Chris Clements.
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