Mullin scored either side of the break in a game where Morecambe striker Cole Stockton missed a vital penalty and the visitors also had a man sent off with Liam Gibson seeing red for a professional foul on 54 minutes.
League leaders Cambridge took the lead in the 17th minute through former Shrimps’ striker Mullin.
Kyle Knoyle delivered a right wing cross to the far post that was met by Joe Ironside. His header was superbly saved by Kyle Letheren but the ball fell nearly for Mullin to poke the ball over the line from close range.
The home side continued to look the more dangerous and went close to a second on 24 minutes when Ironside was again found in the box with a final effort forcing a desperate goal line clearance from the Morecambe defence.
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Morecambe’s first attack of any note came on 29 minutes when Toumani Diagouraga volleyed wide from Ben Pringle’s corner. Moments later the Shrimps hit the home side on break but Cole Stockton failed to fine Aaron Wildig in space.
The Shrimps were almost made to pay moments later when Cambridge hit them with a swift counter attack as Mullin’s cross field ball found Joe Iredale in the box only for the left back’s shot to be superbly saved by Letheren at his near post.
The Shrimps were much better after the break and dominated the opening period until Gibson was sent off for pulling back Mullin on 54 minutes as he looked to get onto the end of a long through ball.
Despite going a man down the Shrimps then enjoyed their best period of the game and were awarded a penalty on 64 minutes when Greg Taylor brought down Aaron Wildig in the box only for Stockton to see his spot kick well saved by Callum Burton.
Carlos Mendes-Gomes then went close to forging an opening of his own after a neat ball into the box before the Shrimps conceded a second when the referee gave a second penalty of the afternoon when Ryan Cooney was harshly adjudged to have brought town Shilow Tracey and Mullin made no mistake from the spot.
To their credit the Shrimps again fought to the death and pulled on back three minutes from time when young substitute Freddie Price headed home from close range after Burton had saved a header but despite ending the game on the front foot time ran out and Cambridge were able to take the points.
Cambridge: Burton, Knoyle, Taylor, Iredale, O’Neil (rep Tracey 75), Okedina, Digby, Boateng, Hoolahan (rep Dunk 90), Mullin (rep Hannant 87), Ironside. Subs not used: McKenzie-Lyle, Drysdale, Alese, May.
Morecambe: Letheren, Mellor (rep Hendrie 58), Davis, Lavelle, Gibson, Diagouraga (rep Denny 87), Wildig, O’Sullivan, Pringle (rep Cooney 58), Mendes-Gomes (rep Price 87), Stockton (rep McAlinden 75). Subs not used: Halstead, Knight-Percival.
Ref: D Rock.