The 1-0 victory over Nottingham Forest on Tuesday night guaranteed that the Royals will finish in the Championship table's automatic promotion places and avoid the play-offs, where they lost in last year's final to Swansea City.
A jubilant McDermott said: "This is my best moment in football. My worst was on the pitch at Wembley on May 30 [after the defeat to Swansea].
"We are in the Premier League now and I can celebrate for the next two months. I need to get away for a while in the summer and have a break.
"Tonight was the most important three points we have got this season because it has seen us over the line.
"I don't know how I am supposed to be. I had the worst summer last season after losing the play off final against Swansea but this will be a good summer."