The Welsh side snatched the lead in the 41st minute through on-loan Millwall forward Neil Harris before Cardiff skipper Graham Kavanagh was handed a straight red card for a vicious tackle on Jon Harley.
Cardiff manager Lennie Lawrence admitted Kavanagh cost the Welsh side the game after the big midfielder was handed his second red card in three matches.
Kavanagh had been lucky to escape with a caution after scything down Alan Quinn in the 37th minute, before a horrendous tackle on Harley five minutes later left Cardiff defending a one-goal lead.
"The sending off was the turning point," said Lawrence. "We were in total control of the game in the first half. I was convinced we would get something.
"He has apologised in front of his team-mates and I won't castigate him publicly.
"The referee got one thing right and that was the sending off. Graham has got to look at himself and his own game and we have got to play without him."
United, with just one defeat in 16 games and with new signing Danny Cullip making his debut after signing from Brighton in a £400,000 deal, were caught out in the opening minutes
Cameron Jerome blundered as he stretched to meet Harris' precise cross and steered his header wide, before Phil Jagielka's misplaced back pass let in Jerome who was robbed by keeper Ian Bennett as he charged out of his area to clear.
United striker Andy Gray headed over from Leigh Bromby's long throw before Cardiff found the net in the 18th minute only to have the goal disallowed. Referee Andre Marriner ruling Peter Thorne pushed Chris Morgan before hitting the target.
Visiting keeper Tony Warner foiled Harley before Cardiff snatched the lead four minutes ahead of half-time. Harris linked cleverly with Thorne before beating Bennett with an angled drive from ten yards.
A minute later Cardiff were down to ten men, before Neil Warnock made two half-time substitutions to turn the game round.
Michael Tonge skimmed the top of the bar with a spectacular shot from long range before United were back on level terms in the 69th minute. Harley's cross was touched on by Gray and substitute Andy Liddell finished off at the far post.
Warner conjured up a stunning save in the 76th minute to push Jagielka's fierce strike onto the post before Gray snatched the winner 11 minutes from time - finishing off from close range after Chris Morgan had headed Liddell's cross back into the goalmouth.