Shane Long swept home Jimmy Kebe's low cross to open the scoring in the 57th minute and then won a penalty which Ian Harte converted to send Scunthorpe closer to relegation.
Reading now lie in fifth spot - just three points behind second-placed Cardiff - and McDermott said: "It was a great result and we're absolutely delighted.
"Scunthorpe played some good football in the first half but we bossed the game after the break and probably deserved to win.
"It's been a decent week but to get three points at Nottingham Forest on Saturday and Scunthorpe is a good few days for us. They were big wins."
Source: PA
Source: PA