Craig Nelthorpe hit his first goal for the club as Oxford United chalked up their seventh successive home win in the league.
It was another impressive U's performance, even if they didn't quite match their attacking play of last week against Barrow.
But that was largely because Mansfield, themselves unbeaten in six games under new boss David Holdsworth before this match, were a tough not to crack.
Nelthorpe's matchwinner came on the hour after some brilliant play by the irrepressible James Constable, who had another outstanding game.
Constable got around left back Paul Mayo with sheer determination, and he picked out his pass to perfection with a square ball, across the face of the goal to Nelthorpe, who hammered home from five yards. Nelthorpe had hit the bar just before half-time, while Craig Farrell blazed over from a very good position just after, and Farrell also went close late on when keeper Alan Marriott saved well.
At the other end, Billy Turley was in great form, keeping to his word in stgopping ex-U's striker Rob Duffy from scoring.
Chris Wilder made the expected change, with Farrell coming in up front to take over from the suspended Lewis Haldane, and Adam Chapman switching to the right of midfield.
Mansfield won the toss and chose to attack towards the open end in the first half, which wasn't what United wanted, and it made it even more essential that the U's got a head of steam going towards their own fans in the first half.
That didn't really happen in the early exchanges, although Constable's combative battling for the ball immediately caused the visitors problems.
And when Constable dribbled past two defenders inside the box to get the game's first real opportunity on ten minutes, only a good blocking save from Stags keeper Alan Marriott kept the scores level.
By then Holdsworth's team had already tested Billy Turley from a 20-yard free-kick.
Chris Willmott gave the free-kickaway, to stop Louis Briscoe after a mazy dribble past two players, and United's players were remembering how Nathan Arnold had scored from a free-kick against them at Field Mill in December.
Again he directed the kick up and over the wall, but Turley was comfortably behind it.
United were probing for an opening, but too often Adam Murray was looking for the killer pass when a simple short option might have been been better.
There were some good pieces of play in other areas from Oxford though.
Chris Carruthers made a number of telling tackles, Chris Willmott was dominant in the air, and Turley got huge distance with a punch under pressure.
Mansfield's Jonathan D'Laryea and substitute Aaron O'Connor both tried their luck with long-range drives, but Turley was troubled by neither.
However, United's keeper needed to be at his best to pull off a superb reaction save from O'Connor's point-blank header on 32 minutes.
Clear-cut opportunities were at a premium but Oxford carved out a couple of half-chances before the break.
Chapman played a neat one-two and scooped his shot narrowly over the bar, and Constable's strong battling for the ball earned him a sniff, but he could only direct his left-footed drive wide.
Moments before half-time, though, Nelthorpe, who had not had the best of starts to the match, got past his full back, cutting in menacingly from the left, and shot against the crossbar, with the follow-up then turned for a corner.
Constable was lucky not to get booked for diving just before United got their breakthrough goal, the centre forward falling theatrically forward when there had been minimal contact.
Farrell went through on goal when he beat the visitors' offside trap, but Marriott saved his shot.
There was always anxiety because Mansfield looked well capable of getting an equaliser.
Turley was smartly off his line to save at O'Connor's feet, and Luke Foster made a magnificent block to prevent Arnold from scoring.
But Oxford saw out the game professionally in the last ten minutes, and Odubade might even have doubled the score, racing through only to slice his shot.
Nevertheless, it was a vital three points, and the team's first double of the season.
Wilder replaced Nelthorpe with Yemi Odubade for the last 15 minutes, with Chapman going into the centre.
Oxford Utd: Turley; Batt, Willmott, Foster, Carruthers; Chapman, Clist, Murray, Nelthorpe (Odubade 75); Farrell, Constable. Subs not used: Hinchliffe, Hutchinson, Burnell, Clarke.
Booked: Murray.
Att: 4,618 (215 from Mansfield)
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