A tense affair which saw few chances of note for either team finished goalless after 120 minutes, and it took the agony and ecstasy of a penalty shoot-out to separate the two in the end.
After Reading had taken an advantage through Ali Al-Habsi's save, back-to-back misses from Royals Liam Moore and Jordan Obita allowed German defender Christopher Schindler to roll in the decisive penalty.
Wagner delighted to make history
The Terriers will start next season in the top division of English football for the first time in almost half a century, and Wagner, soaked with champagne, revelled in the magnitude of his side's achievement.
"I think everybody know the last time this club was in the top flight was 45 years ago. This is a big, big achievement and I am so so happy for my players, my group, the whole town who helped Huddersfield take these steps forward.
"Especially for the chairman who backed nearly all of my ideas, even when he thought maybe they were ridiculous. He said, 'If you are sure, let's do it.'
"Now we've done it. We said no limits, now we know where our limit is. The limit is in the Premier League. We are very happy. This is a very hard competition, and my thoughts are with the other three teams [in the play-offs] as well."
Critics silenced by 'inexperienced' Wagner
Wagner had his doubters when he took over the Yorkshire club last season, but has silenced them with a season of unexpected success after avoiding relegation last season.
Some of his methods have been unconventional - a three-day survival camp in pre-season raised eyebrows - but Wagner has now proven his pedigree as a young manager of some talent, and he will continue to make his name in the Premier League next season.
"We said we wanted to leave our comfort zone to be competitive. I asked the players a lot of times to do this and they did it, it's great that they get paid back something for what they invested in.
"We wanted to find new ways to be competitive, I'm so happy we've proven that experience is not everything, particularly here in England. Since I arrived, I was always confronted with 'no experience, no experience of English football, no experience without the winter break, no experience of the play-offs'.
"Experience is important, but if you have passion behind an idea, you can match it. I'm so happy we've shown this is possible, that we've brought this fairy tale to a happy conclusion."