Damian Batt came up with his first goal for the club as league leaders Oxford United continued their superb home form.
Matt Green had missed a penalty and several others struggled to find a way through or past Altrincham keeper Stuart Coburn.
But, for the second week running, the introduction of Sam Deering lifted the U's, and it was after Deering had come so close to engineering a breakthrough, with a driven right-wing centre across the face of the goal that Green was just unable to reach, that Chris Wilder's men took the lead.
Steven Kinniburgh cleverly beat his man to cut the ball back from the bye-line, and right back Batt drove a 20-yard shot into the ground with such power that it then took opff and flew past Coburn before he knew what had happened.
Altrincham were without three players - Tom Kearney, Chris Senior and James Smith - through suspension, and the three-match ban controversially handed out to Smith after a video posting on You Tube had left them very aggrieved.
Stuart Coburn denied Oxford with a fine penalty save.
Simon Clist won the 11th-minute spot kick when he was brought down by Coburn after a forceful run by Matt Green.
The keeper was booked for the foul, but when Green took the penalty, he redeemed himself by diving to his right to beat out the shot.
Coburn had also come bravely off his line to save Green's glancing header at almost point-blank range.
Altrincham had to defend for most of the first half so when rare half-chances came their way, they really needed to take them.The ball arrived unexpectedly at Colin Little's feet when Luke Foster missed his clearance, but the Altrincham striker was taken unawares and it ran away from him in front of goal.
Constable became more and more frustrated after he was penalised for a foul on the keeper, and the ref spoke to him several times about his backchat before Oxford's leading scorer was eventually yellow-carded, for the sixth time this season.
Kent referee Ian Cooper produced some baffling decisions, which got the home fans worked up, but it was a linesman who made a key decision just before the break, raising his flag when Jack Midson scored to rule him offside.
That only added to Midson's frustration, for a few minutes earlier, he nodded Steve Kinniburgh's cross over the bar from a great position.
Damian Batt delivered an excellent right-wing cross nable to convert that his teammates were just at the start of the second half as the U's continued to dominate.
And a driving run through the middle by Green ended with Constable slightly scuffing his left-foot shot, giving Coburn an easy save by his near post.
Deering's arrival seemed to give United better balance, and after Batt's breakthrough, it looked like other goals would follow.
Green fired into the side-netting, Deering drove over and Constable had a late volley saved.
In the end, though, it needed some fine work at the other end for Oxford to extend their record at home this season, in league and Cup, to eight wins and one draw from nine matches.
Shaun Densmore had the goal at his mercy beyond the far post after Aaron Burns had beaten Foster, but Kinniburgh got back onto his goalline and chested Densmore's shot away for a corner.
Oxford Utd: (4-3-3) Clarke; Batt, Foster, Creighton, Kinniburgh; Bulman, Murray (Perry 86), Clist; Midson (Deering 61), Constable, Green (Cook 77). Subs not used: Turley, Sandwith.
Referee: Ian Cooper (Kent) Att: 5,609 (79 from Altrincham)
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