Mullins’ wonder-goal came after a game that was largely devoid of clear cut scoring opportunities, with both sides struggling to take control of the match on what proved to be a difficult pitch.
Story of the match
Promotion chasing Cambridge came into the game looking to regain their place at the top of League Two, but it was the out of form Saddlers who showed the greater attacking intent following the kick off.
Emmanuel Osadebe came close to giving the Staffordshire side the lead in the 22nd minute after he chested down a Cameron Norman corner and produced a brilliant overhead kick which was just tipped over the bar by goalkeeper Callum Burton.
Osadebe then again nearly opened the scoring when he skilfully took the ball past two Cambridge defenders in the 35th minute, but could only hit the side netting.
Tyreik Wright also came close to giving Walsall the lead after 40 minutes when he was slipped in behind by the dangerous Osadebe, with his effort curling just wide the back post.
United began to take a greater control of the game following the break however, coming close to taking the lead just seconds after the restart when Joe Ironsides glancing header from a Mullins cross forced a good save from Liam Roberts.
Their were few clear cut chances for either side in what proved to be a scrappy second half though, Adam May coming closest to breaking to dead lock when he missed a great opportunity for Cambridge after his low shot bounced just wide after being played in on the edge of the box by Wes Hoolahan.
The game seemed to be destined for a goalless draw, however League Two's top-scorer Paul Mullin produced a sensational 93rd minute strike from 25 yards out to give Mark Bonners side all three points.
The victory sees Cambridge remain in second place, one point behind leaders Cheltenham, while Walsall drop down to 18th.
Man of the match - Paul Mullin
In a game where neither side was able to fully take control and where chances were few and far between, Mullin created the one piece of magic that was able to separate the sides.
His stoppage time stunner, his 23rd goal of the season, was undoubtedly the stand out moment in a game lacking in quality.