Mullins drilled low into the net from 15 yards after James Constable knocked the ball back to him to keep the U’s unbeaten run intact.
The Chairboys were on top in the first half and took the lead when Dean Morgan nodded home in the 42nd minute after Gary Doherty’s deep cross was headed on by debutant Billy Knott.
Wycombe were unfortunate not to be two up at the break as Ryan Clarke clawed the ball away from his goal after racing back to stop Neil Arnold’s 40-yard lob from going in.
Oxford brought on Dave Kitson for the second half which began in dramatic fashion. Wycombe left-back Charles Dunne was shown the red card for use of an elbow in the 49th minute as Alfie Potter tried to sprint past him.
And from Danny Rose’s free kick, the U’s were on level terms as goalkeeper Matt Ingram misjudged it and saw the ball bounce over him and into the goal.
But that turn-around was brief. David Hunt’s rash tackle on Morgan brought the visitors a 59th minute penalty, and Morgan converted the spot kick into the bottom left-hand corner for his second goal of the game before the final dramatic twist.