The Stags, who were four unbeaten on the road, turned in a fine display over the 90 minutes, but the U’s gave them a deadly lesson in finishing to extend their unbeaten run to eight.
Sam Clucas, Matt Rhead, Ryan Tafazolli and Lee Stevenson all had good chances they failed to take.
But, after David Connolly had netted a soft goal against the run of play on his full debut for United on 27 minutes, it was always an uphill battle for Paul Cox’s men who need someone to finish off the chances they are carving out.
Connolly was the architect of Portsmouth’s comeback against Mansfield last month and, having moved on loan, was back to haunt the Stags today.
Nicky Wroe then made it 2-0 with an absolute pearler from outside the box on 53 minutes before Deane Smalley completed the scoring with a follow-up close range finish on 68 minutes after Alan Marriott had blocked a James Constable effort.
The win left Oxford third in the table, four points off the top, with Stags dropping a place to 18th.
Mansfield were unchanged, but reserve keeper Ian Deakin replaced striker Ross Dyer on the bench.
The game survived an early morning pitch inspection after yesterday’s heavy rainfall.
It was clear from the start that Stags had a blustering gale very much behind them as Oxford struggled to kick clear, though that eased as the half wore on.
The home side also had an early Jennings long throw and a dangerous Clements free kick to deal with.
A slip by Bevans allowed Clucas to race into the box on the left on seven minutes, but his cross was behind his waiting team mates and, after a scramble, Oxford cleared their lines.
A good surge down the left by Newey brought Oxford’s first threat on 10 minutes. He fed Kitson, who managed to get past Riley but, from a tight angle, sliced a finish horribly wide of the far post.
Jennings took a clattering challenge in Mansfield’s next raid and stayed down. The physio signalled to the bench that Jennings was not going to be able to continue and, after a minute or two with 10 men, Stags were finally able to get Daniel on as replacement.
Clucas forced a good low save out of Clarke at his near post on 18 minutes when Oxford failed to clear a Daniel throw, and Clements then saw his inswinging corner punched away by the keeper.
Two minutes later Stags came close again as Westlake’s pass found Howell on the far side of the box. He drilled the ball in hard and low and Rhead’s attempted sidefoot finish was blocked and Stevenson saw his powerful low follow-up deflect wide for a corner.
There was another hold-up as Hunt was treated for a knock in that melee.
But when Clements finally got his corner over, it was cleared out to Westlake who launched the ball back in.
Rhead headed it to his right and Clucas, with only the keeper to beat, met it with a first time finish that Clarje did well to beat away from 10 yards.
Mansfield were well on top at this stage and soon after Clements was narrowly wide with a low shot from 20 yards.
So it was totally against the run of play when the home side grabbed the lead on 27 minutes.
Newey played the ball in from the left to Connolly six yards from goal who turned well to scuff in a low shot at goal that Marriott looked to have covered but allowed to drift under his dive and into the net.
Bevans got in a good block on a Clucas finish after Rhead and Stevenson had helped on a long Dempster ball.
But on 38 minutes a Riley clearance bounced perfectly for Constable 25 yards from goal and he smashed a powerful half-volley at goal. Thankfully Marriott was equal to it, diving to his right to turn it aside.
Stevenson tried something far more ambitious on 42 minutes as the ball bounced 40 yards out and he tried to lift it over the keeper with the help of the wind only to see it sail over the bar.
Ex-Stag Mullins was pole-axed by a Stevenson shot that hit him in the head from close range on the edge of the box which gave us yet another injury hold-up and four minutes of added time.
Home fans were angry when Kitson was penalised for a foul on Tafazolli. The home side then struggled to clear Clements’ free kick and allowed the ball to drop invitingly for Stevenson who poked a finish just wide of the near post when he probably should have hit the target from 12 yards.
Even added time brought another injury as Riley went down and eventually hobbled off as the whistle finally blew on a half that Mansfield had dominated but went in trailing 1-0.
Riley was unable to return so he was replaced by McGuire for the second half, Stags changing to a back four.
Within two minutes Clucas guided a half-volley wide of the far post from the edge of of the Oxford box on the right.
At the other end Constable was too high from Connolly’s pass.
But Oxford doubled their lead out of nothing on 53 minutes.
Wroe looked up from 20 yards out in a central position and picked out the top right corner with a sensational finish that gave Marriott no chance.
Stags almost pulled one back straight away as Clucas met Clements’ cross with a rasping finish that floored Bevans as he blocked.
He stayed down as play went on, though jumped up at one stage to try to clear again to the anger of the away fans, the pressure ended when Stevenson bundled a low shot narrowly wide.
On 56 minutes Rhead sent a close range header at Clarke from a right wing cross as Mansfield continued to press.
Four minutes later yet another chance went begging for the visitors as Clements floated in a free kick from the left and Tafazolli sent a free six yard header straight at Clarke.
McGuire saw the afternoon’s first yellow card for going in late on Kitson on 63 minutes.
A minute later Oxford won their first corner of the game which Stags eventually cleared after Marriott initially came for it but failed to reach.
Kitson joined McGuire in the book on 66 minutes for going in late and hard on Stevenson.
Westlake tried to police Constable on the edge of the box, but the powerful striker managed to turn him and lash a shot just over Marriott’s crossbar.
Stags found themselves three behind on 68 minutes.
Constable burst into the box on the left via a one-two with Connolly and fired in a low shot from a tight angle that Marriott managed to block, but Smalley was on hand to turn the loose ball into the net.
Two minutes later Westlake got to the right by-line and cross to Clucas who turned the ball straight at Clarke, summing up the afternoon.
Smalley tried a long range half-volley straight at Marriott as the home side finally began to relax and enjoy their afternoon.
Howell’s dive in the box on 78 minutes rightly earned him a yellow card and boos from the home fans.
Williams took over from Connolly for the last 12 minutes and quickly tried his luck from distance but it lacked enough power to trouble Marriott.
Howell was replaced by Palmer on 84 minutes as Stags desperately looked for even a consolation goal.
Rhead was added to the book on 86 minutes for jumping into Clarke as the keeper claimed a cross.
Stevenson was well over from 20 yards as the visitors gamely kept pouring forward. Then Daniel caught the ball cleanly with power from 25 yards, but again it flew straight at Clarke.
Westlake then drilled a low shot across the face of the goal with the final kick of the afternoon.
OXFORD: Clarke, Newey, Kitson, Constable (Potter 90+1), Smalley, Hunt, Rose, Mullins, Wroe, Connolly (Williams 78), Bevans. Subs not used: Henly, Marsh, Long, O’Dowda, Ruffels. STAGS: Marriott; Dempster, Tafazolli, Riley (McGuire HT); Westlake, Howell (Palmer 84), Stevenson, Clements, Jennings (Daniel 17); Rhead, Clucas. Subs not used: Meikle, Hutchinson, Sutton, Deakin.
REFEREE: Fred Graham of Essex.
ATTENDANCE: 5,108. CHAD STAGS MAN OF THE MATCH: Sam Clucas.