A draw would have been enough to secure a play-off place for McClaren, who has been strongly linked with Newcastle, but Reading made a nonsense of their recent poor form to outplay Derby after Darren Bent missed his first penalty since April 2010.
Reading had lost their four previous Championship games but they were ahead in the second minute when Kwesi Appiah intercepted a loose pass from Will Hughes and raced away to go past Richard Keogh and shoot low past Lee Grant.
Derby had a great chance to level in first-half stoppage time when Jordan Obita hauled down Johnny Russell but Bent’s spot-kick was superbly saved by Adam Federici plunging low to his right.
The second half was all Reading, who defended with determination and hit Derby on the break to complete a deserved victory with two more goals.
Michael Hector smashed in a loose ball when Derby failed to clear a corner in the 72nd minute and Garath McCleary sealed Derby’s fate five minutes from time from the spot after Grant had tripped Jem Karacan.
It left McClaren to reflect on what went wrong after Derby were five points clear of third place at the end of February.
“I’m really annoyed, angry, and frustrated about the last two months and really determined to do something about it,” he said.
“I’m not making a pledge, this is how I feel at the present moment. I’m under contract at Derby and I’ve said that all along. I’ve been questioned about all the speculation which has been going on for about six months.
“My intention is to sort this out. No one has told me otherwise and until somebody does, that’s my determination.
“I’m angry, frustrated, I want to do something about it. I want to sit down with everybody next week, I think it’s too raw, too early to make any kind of statement but what I say is what I mean.”
On the speculation linking him with Newcastle, McClaren insisted: “Absolutely nothing has been agreed, absolutely nothing.
“Over the next two days we’ve got to sit down with this club (Derby County) and put it right. I’m a part of that process and I want to be a part of that process and that’s what I’m going to do.”
For Reading boss Steve Clarke, the result was overdue.
“A lot of performances have been really good but the key moments in the game went against us but today, the key moments in the game went for us and that’s the difference,” he said.
“The players were stuck in a little bit of a rut and had forgotten how to win a game and today they managed to remember so I’ve told them when they go away for the summer, don’t forget what it’s like to win a game! So this is a good way for us to finish.”
Clarke was delighted for Federici who followed his Reading Player of the Year award with that crucial penalty save. “I was pleased for him when he saved the penalty because he was getting a lot of stick off the Derby supporters and sometimes when you get that it’s nice to give a bit of retribution back,” he said.