Castleford won the League Leaders' shield but were unable to win rugby league's biggest prize as they were defeated 24-6 by the Rhinos in the Manchester showpiece finale.
Danny McGuire produced the perfect swansong, leading Leeds to victory on his final appearance for his home-town club.
The Rhinos skipper scored two tries, helped to create others for winger Tom Briscoe and landed two drop goals in a man-of-the-match performance.
"That's the final chapter of the book," McDermott told Sky Sports arena. "For us to make the Grand Final is a great story and we've just capped off the final chapter.
"I have been saying to my players all week that you don't get many chances to have moments in life that stand out where every man, jack and his dog up the country will think, 'They can't go and win the Grand Final can they? Not from where they were'.
"We've done it. That group of people over there have gone through every emotion going in the last 18 months, the whole club.
"This isn't just a coach's win or the players' win. Everybody at Leeds Rhinos, including the fans, who stood with us when the dark times were really, really dark, this is everybody's. The whole club deserves this.
"Never did I dream in my wildest dreams that we'd get to Old Trafford never mind win it. Now look at us.
"It's the best win ever this one."
Castleford coach Daryl Powell was left to rue a slew of handling errors by the Tigers.
"We saved our very worst 'til last," Powell said.
"We just created too many errors and credit to Leeds, they deserved to win.
"We just made unforced errors, mistakes and dropping the ball is so hugely disappointing.
"You have to wait a long time before we can straighten it up but it is what it is.
"We've come to a big occasion and we made 18 errors and you can't win a Grand Final playing like that."