Mark Beesley's double strike sent Cambridge to the top of the Blue Square Premiership and consigned Wrexham to their third defeat of the season.
Beesley's opener came after 28 minutes, as he fired Mark Convery's corner into the top right-hand corner.
Beesley's second also came from close range after Chris Holroyd's shot hit the bar and rebounded to give the former Chester player his chance.
Wrexham began promisingly, but failed to make an impact.
Wrexham manager Brian Little:
"I thought we started well enough, but then gave them two bad goals and we never really looked like getting back into the game.
"That was the disappointing thing - that we really never functioned, certainly not in the final third.
"We were okay for the first 15 minutes, but once the goal went in, I'm not sure if we lost desire, but we didn't look like getting back into the game.
"We've had a little talk about that which is the right thing to do, but generally collectively and individually the team were below what we would like of them."
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