Nathan Collier snatched a valuable win for Dartford with a deflected shot in a hectic finish at the Abbey Stadium.
Collier's effort looped over debutant keeper Craig Ross soon after James Jennings's penalty appeared to have rescued a point for Cambridge.
Danny Harris gave Dartford a fortuitous lead soon after the break when his cross eluded everyone, including Ross.
Jennings levelled after Sam Smith was fouled in the box but Dartford were not to be denied as Collier found the net.
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Cambridge United manager Jez George told BBC Radio Cambridgeshire:
"We weren't good enough, in the first shape we had, to keep the ball, to pass the ball and create chances.
"We had to go a little bit gung-ho in the end and the little spell before and after the equaliser was probably our best passage of play.
"I take responsibility for the second goal because probably I should just have been thankful for the equaliser and tightened it up again, played four at the back and made sure we got something.
"But I felt we had the momentum, the crowd were with us, we'd scored the goal and two or three other good chances and that we could go on and win the game."
Cambridge United 24 Ross 03 Jennings 12 McAuley 14 Garner (Hughes - 70' ) 25 Hudson 08 Wellard 11 Dunk (Gibson - 79' ) 18 Berry 21 Moke (Smith - 58' ) 22 Willmott Booked 10 Elliott Substitutes 04 Coulson 07 Gibson 16 Jarvis 17 Hughes 28 Smith Dartford 12 Bettinelli 02 Rose 04 Champion 06 Bonner 16 Arber 08 Wallis (Noble - 81' ) 20 Burns 07 Hayes (Collier - 69' ) 09 Erskine (Crawford - 87' ) 10 Bradbrook Booked 11 Harris Substitutes 24 Sutton 13 Rogers 14 Noble 15 Collier 21 Crawford