Rangers were clinging on for dear life when substitute Bobby Zamora broke Derby's hearts with a last-minute Wembley winner.
It was Rangers' first and only shot on target but it will be worth an estimated B£120million to the west London club, who secured promotion back into the Premier League at the first attempt.
It was a sad return to Wembley for Rams boss McClaren, back at the scene of his lowest moment in football when he cowered under an umbrella as his England side crumbled against Croatia.
Despite more rain McClaren, who began the season on the coaching staff under Redknapp at QPR, wisely opted to leave the brolly at home this time.
His young side dominated the match for 89 minutes but could not find a way past Robert Green and were eventually sunk by a late sucker punch.
"Football's a cruel game," said McClaren. "We were dominating but one lapse and you lose a game like that.
"I've told the players we obviously are not ready yet, that it's not our time yet and we still have a lot of learning to do, but I'm so proud of them.
"We stayed on the field to applaud QPR and watch them go into the Premier League, and we don't want that hurt again. But if this team stays together it will get stronger."
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