On a tense afternoon at Wembley, QPR had to withstand a Derby onslaught in the second half after Gary O'Neil's straight red card for a foul on Johnny Russell.
But Zamora, who hit a play-off winner for West Ham in 2005, pounced in the final minute to dash Derby's dreams and secure a return to the top flight for QPR just a year after being relegated.
It meant a sad return to Wembley for Rams boss Steve McClaren, back at the scene of his lowest moment in football when he sheltered under an umbrella as his England side crumbled against Croatia in 2007 to miss out on qualification for the following year's European Championship.
McClaren, who began the season on Rangers' coaching staff, must have had a wry smile on his face when, 10 minutes before kick-off, the heavens opened.
All the pressure was on Rangers, £177million in debt and with a wage bill bigger than Atletico Madrid's, and it showed.
Derby bossed the first half and Craig Forsyth headed a deep cross from Jamie Ward narrowly wide, while Will Hughes blazed over from the edge of the area.
Rangers' hearts were in their mouths in the 27th minute when Hughes turned inside Richard Dunne in the area and promptly fell to the ground, but referee Lee Mason waved away the appeals for a penalty.
Niko Kranjcar suffered a hamstring injury and the man who helped hasten McClaren's England exit with his goal for Croatia that infamous night seven years ago limped off to be replaced by Armond Traore.
Rangers keeper Rob Green dealt with Russell's deflected effort and also got down well to tip Ward's inswinging free-kick, which evaded everyone in the six-yard box, past the post.
Rangers boss Harry Redknapp was pinning his side's promotion hopes on their top scorer Charlie Austin, and the chance the former Burnley striker had been looking for arrived shortly before the hour mark.
Kevin Doyle sent Traore down the left flank and his cut back found Austin where he has been lurking most of the season, in front of goal 10 yards out.
But this time the 20-goal frontman's finish let him down as he side-footed the ball wide of Lee Grant's goal and held his head in his hands with disbelief.
It looked like being a defining moment for Rangers, and worse was to follow three minutes later when Russell's attempt to burst through on goal was curtailed by a cynical and deliberate foul from O'Neil.
O'Neil argued that there were other defenders in the vicinity but referee Mason, probably wisely, decided veterans Dunne and Clint Hill were not going to catch Russell and produced a straight red card.
Derby sensed their chance and poured forward, forcing Green into point-blank saves from Craig Bryson, Chris Martin and Simon Dawkins in a frantic 10-minute spell.
But as the clock ticked down Derby skipper Richard Keogh mis-controlled in the area and Zamora, for the second time in his career, blasted in the most valuable goal in football.