The U's had not won a game this season and had just a solitary point from their previous four games. The Daggers on the other hand are comfortably in the top half of the table losing just one game this season.
The home side started the game brightly and dominated the early possession. In the second minute Danny Foster made a run down the left, but his cross could not pick out Paul Benson.
The visitors tried to get hold of the ball and create chances of their own and despite Marcus Richardson and debutant Michael Gash linking well, they lacked the support of there team-mates.
The first real chance of the game fell to Dagenham after 15 minutes. Sam Saunders' cross from the left was missed by the Cambridge defence and fell to Benson, who skied his shot over the bar from six yards.
Three minutes later Cambridge had an even better chance. A corner from the right was flicked on by Matt Bloomer to an unmarked Richardson, but his diving header brought a brilliant point-blank save from Tony Roberts.
The home side hit the woodwork just after the half hour when Saunders' shot from the right of the area hit the post and fell the feet of Mackail-Smith, who forced Paul Crichton to make a good save.
The U's tried to take an early advantage in the second half but midfielder Darren Quinton shot wide from 20 yards. Moments later Quinton played Gash through.
The forward was one-on-one with Roberts but he failed to get the ball under control and spurned his chance.
That was the best Cambridge could produce as Dagenham went on to dominate the rest of the game.
Just before the hour Mackail-Smith played Benson in and his shot from ten yards was parried for a corner by Crichton.
Benson then had another chance to open the scoring when he was unmarked at the far post from a Glen Southam cross, but his header produced another good save from the veteran Cambridge keeper.
The deadlock was finally broken on 64 minutes when Benson headed in a Saunders corner, despite the best efforts of Stephen Smith to keep it out.
The lead was doubled four minutes later when Benson broke with the ball and played Sam Sloma in on the left of the box and his low powerful drive left Crichton with no chance.
Things got worse for the visitors five minutes from the end when Tommy Jaszczun was sent off for a clash with Saunders.