The Royals were far from their fluent best for much of the encounter and fell behind to an early goal from Posh's Tommy Rowe. Simon Church tapped in a close-range equaliser for the hosts, but the match seemed to be meandering towards a dull draw until the introduction of Le Fondre and Robson-Kanu midway through the second half.
Both netted within two minutes of each other in the latter stages of the contest and, while Emile Sinclair grabbed a late consolation, Reading were able to celebrate three npower Championship points.
"The two subs probably changed the game for us," said McDermott. "They came off the bench to make the difference. We didn't feel we were at our best but we got three points and that was what it was about.
"I'm pleased with the result. I thought Peterborough were very good and caused us a lot of problems in the first half. We lacked a little bit of energy in the first half but we were better in the second half.
"You've got to credit Peterborough. I thought they were very good. They will be upset they've lost the game, I'm sure. They've got a side that's going to cause everyone in this division problems, there's no doubt about that."