Morgan twice gave Wanderers the lead, who saw Charles Dunne sent off early in the second half, but Mullins drove home after what looked to be a push on Sam Wood.
It was fine performance from Gareth Ainsworth’s side who outplayed a side who started the day on top of Sky Bet League 2 for large periods and then defended heroically with 10 men.
Ainsworth made five changes to his side with Billy Knott coming in for his debut as well Leon Johnson, Sam Wood, Dean Morgan and Steven Craig. The men to be replaced were Anthony Stewart, Matt Spring, Matt McClure, Jo Kuffour and the injured Paris Cowan-Hall.
Former Chairboy Scott Davies had the first attempt on goal as the hosts started brightly, however, his third-minute strike was straight at Matt Ingram who made a comfortable stop.
Wanderers settled well after that and Nick Arnold was just unable to get on the end of Johnson’s flick-on from Josh Scowen’s free-kick.
Morgan saw a volley deflected over after Gary Doherty had helped on Marvin McCoy’s long throw as the combination of Knott and Scowen enjoyed the upper hand in the centre of the pitch.
Knott, who wanted the ball as often as possible, released Morgan in the 20th minute with a pin-point ball over the top, but Jake Wright recovered well to scramble away.
It was that man Knott who was involved again on the half-hour when he closed down Ryan Clarke’s clearance, but luckily for Oxford’s keeper the ball bounced harmlessly out for a throw.
Morgan almost put Craig through on goal a minute later after turning his man, but his pass had slightly too much on it and the ball ran through to Clarke.
Dunne fired over from distance before James Constable turned Doherty on the edge of the area, only for his low shot to trickle through to Ingram who saved low to his right.
Chairboys got the goal their first half display deserved four minutes before the break when Morgan headed home unmarked from six yards. Doherty knocked the ball forward and Knott headed on for Morgan, who glanced into the far corner, giving Clarke no chance.
It was so close to being two a minute later when Oxford again failed to clear convincingly and Arnold tried to chip Clarke from 30 yards. However, the keeper recovered superbly well to claw away and deny the Reading loanee a stunning first professional goal.
Ainsworth wouldn’t have wanted the half-time whistle to come as his side were firmly in control of the game, having nullified the threat of the league leaders in the opening 45 minutes.
The hosts introduced Dave Kitson for Davies at the break and were given a boost five minutes later when Dunne was shown a straight red card by referee Andy D’Urso for an alleged elbow on Potter as he looked to fend the winger off.
Oxford took advantage of their numerical supremacy immediately as Danny Rose’s free-kick from near the touchline evaded everyone and beat Ingram at the far post.
The hosts began to increase their tempo, but Blues were still a threat on the break and retook the lead just before the hour.
The irrepressible Morgan received the ball in the area and tricked his way past David Hunt before being hauled down by the right-back. The forward dusted himself down and blasted the resulting spot-kick past Clarke.
Ainsworth made his first change in the68th minute bringing on former Oxford man Jon-Paul Pittman for Craig, who ran his socks off for the Blues, especially after the sending off when he was up front on his own.
Oxford centre-back Johnny Mullins forced Ingram into a diving save high to his left in the 72nd minute and Blues’ No.1 then claimed the resulting corner to ease the pressure.
Morgan left the pitch 15 minutes from time to a deserved standing ovation with Jo Kuffour replacing him on the left.
Ingram made a stunning stop to deny Kitson with seven minutes remaining after the former Premier-League man blocked McCoy’s clearance and then tried to curl into the far corner from the edge of the area.
Knott was replaced by Spring two minutes time after a superb first performance in the famous quarters.
McCoy threw his body on the line deep into injury-time to block Tom Newey’s powerful drive.
However, Wanderers’ brave defending wasn’t rewarded with the full three points as Mullins drove home six minutes into injury-time after what looked a clear push on Wood in the area.
Wycombe: Ingram, McCoy, Dunne, Johnson, Doherty (c), Arnold, Knott (Spring 88), Scowen, Wood, Morgan (Kuffour 75), Craig (Pittman 68).
Subs: Harrison, Stewart, Kretzschmar, McClure, Kuffour.
Oxford: Clarke, Hunt, Newey, Mullins, Wright (c), Whing, Rose, Hall (Williams 52), Davies (Kitson 46), Potter, Constable.
Subs: Crocombe, Raynes, Marsh, , O'Dowda, Ruffels.
Attendance: 7, 100 (988 Wycombe)