The Chairboys started the day inside the top three but after Southend and Bury each won they will head into the final day of the season in fifth, three points off the pace.
The Shrimps were the better team throughout and Matt Ingram had a great game in the home goal but could do nothing about Mullin’s header after 86 minutes.
After a quiet first half Jack Redshaw found space in the Wycombe box but Ingram made a superb reaction save.
The second period started much brighter, however, and Alfie Mawson looked like he had scored from a corner but it was somehow scrambled away by Ingram.
The Chairboys stopper then made a string of superb saves as Alex Kenyon, Andy Parrish and Kevin Ellison each saw their headers saved.
But inside last five minutes Mullin was found in the box in acres of space and coolly headed in to seal the victory.