It looked as though Cambridge, with three players making their home debuts and reduced to ten men midway through the second half, might cling on for a vital victory.
They were the better side in the first half, taking the lead in the 35th minute when Colin Heath, on loan from Manchester United, set up Richard Hodgson.
But the second half turned into a siege in which the home side could hardly get out of their penalty area after centre-back Warren Goodhind received a red card for a foul on Steve Torpey.
It was Torpey who eventually got the equaliser after a series of near misses by Paul Hayes and Andy Butler. He managed to force the ball in as half a dozen players jostled in the six-yard box in the 87th minute.
The spirit was sucked out of Cambridge, and in the second minute of stoppage time Cleveland Taylor, who had transformed the leaders when he went on as a substitute at the start of the second half, raced away for the winner.
He hammered a shot from the edge of the box and Andy Duncan, who blocked a Hayes header on the line a few minutes earlier, this time could only deflect the ball past his own keeper.
It was a bitter blow for struggling Cambridge, but they paid the penalty for falling back on defence and losing possession even before Goodhind was sent off.