Ismaila Sarr's penalty early in the first half proved to be just enough to earn Watford a 1-0 victory over Millwall and confirm an instant return to the Premier League.
Brentford's win at Bournemouth earlier in the day meant that only three points would be good enough for Xisco Munoz's Hornets to be promoted alongside Norwich, who came down with them last season.
Millwall came close on a number of occasions to delaying a promotion party that traditionally sees Hornets fans jump in a town-centre pond, but Sarr's 11th-minute penalty proved enough to book their place back in the top flight.
Munoz was drafted in midway through the season to take over from Vladimir Ivic and will now plot a way to make sure Watford's Premier League stay is an extended one.
Deeney hails Munoz impact
Watford striker Troy Deeney told Sky Sports:
"He's been great, not only as a coach, but as a human being. He's been brilliant, he deserves all the credit. When you look at earlier games, you can see when managers and players aren't connected, he's got players running through brick walls for him. That's the sign of a great coach. He always puts it on the players, but it's a great time for everybody.
"There's loads of people that deserve it, fair play to the ownership, they could have chopped everybody, sacked everybody during the pandemic, they've kept all the office staff on, but they believe in us and we've taken them back to the Premier League."
Opta stats - Watford join Norwich, Xisco joins Rafa and Co.
- The Championship top two this season are now guaranteed to be Norwich City and Watford, the first time two clubs relegated from the Premier League the previous season have finished in the top two since 2009-10 (Newcastle and West Brom).
- Watford manager Xisco Munoz is the fourth Spanish manager to earn a promotion in Football League history, after Roberto Martinez (2007-08, Swansea City), Aitor Karanka (2015-16, Middlesbrough) and Rafael Benitez (2016-17, Newcastle United).
Fan view: What promotion at the first time of asking means
Mike Duffy, The Voices of the Vic:
Wow, what a season. We came down with expectations of an immediate return to the Premier League but it didn't quite go to plan under Vladimir Ivic and a play-off place was looking like best-case scenario at one stage.
Xisco came in, changed our formation to a 4-3-3 and we just clicked. Nathaniel Chalobah was the star of the show and back to the levels of the young lad we signed on loan from Chelsea in the 2012/13 season. Unbelievable.
Will Hughes back from injury was a massive plus, too. He is such a calm player on the ball. I started believing after the 3-0 home win against Birmingham, though I wouldn't ever admit it.
Looking at our promotion rivals, Swansea and Brentford were perhaps a bit reliant on certain players to score and help them get over the line. I think we edged it over them due to our to all-around togetherness. A real team effort from Watford.
I have to be honest, after the away draw at Coventry in mid-February, I did not expect this. I really can't put into words how this feels at the moment. It's a huge sigh of relief. I personally predicted a fourth-placed finish with a loss in the play-off final so I'm ecstatic and on cloud nine. It's been ages since we tasted promotion and I forgot how sweet it tasted.
Key to success - and planning for the Premier League
Watford played a more attacking style under Munoz, with Ismaila Sarr the key. Will Watford be brave enough to adopt the approach of Sheffield United last season and Leeds this?
In Sarr, Watford have one of the stand-out performers in the Championship and a lot will be expected of him next season.
But the Hornets will need more firepower. Joao Pedro has nine goals, but Troy Deeney and Andre Gray have just 11 between them and Watford will need to eek more out of their attacking options or look for reinforcements.
In addition to a lack of attacking options, some important members of the Watford squad are north of 30.
Key figures such as Deeney and Tom Cleverley are 32 and 31 respectively, while Ben Foster is now 38 and has shared the gloves at times this season with Daniel Bachmann who has no Premier League experience. Craig Cathcart, who played 34 times in the Hornets' relegation year is also 32 and the club will need to decide on what blend of youth and experience they want.
While recruitment will be key, one of Watford's biggest barriers during their last season in the Premier League, and indeed overall for some years, has been the lack of continuity at the helm.
The club had four managers during their last top-flight season, having kicked off the campaign with Javi Gracia. The Spaniard was then replaced by the returning Quique Sanchez Flores, who only himself lasted until December.
Nigel Pearson then took over and, with Watford three points above the drop zone he was sacked with interim manager Hayden Mullins overseeing the final two defeats. If Munoz can escape the sack and have a chance to build something, it could aid Watford's chances.
There is plenty for the Hornets' hierarchy to ponder - but, for now, the club can toast their swift return to the top table.
How Watford earned decisive win
Xisco made a single change to the side that had won at already-promoted Norwich on Tuesday, with Kiko Femenia returning from a ban at right-back to oust Craig Cathcart. Millwall made three changes to the line-up that lost 4-1 at home to Bournemouth the day after.
Jed Wallace forced Watford goalkeeper Daniel Bachmann into a routine save in the opening exchanges that were brought to life by a penalty awarded to the hosts in the 10th minute.
Femenia's burst forward saw the Spaniard exchange passes with Sarr, who was brought down just inside the box on the right by Billy Mitchell's lunge.
Sarr stepped up to slot the spot-kick past goalkeeper Bartosz Bialkowski with ease a minute later.
Millwall looked lethargic but Bialkowski was able to get behind Dan Gosling's header from a free-kick conceded by Mason Bennett's challenge on Sarr.
Joao Pedro saw a shot blocked from another move that opened up the Lions and Will Hughes was only just off target from a half-cleared corner.
Set-pieces seemed the most likely source of Millwall retaliation, with Tom Bradshaw firing just wide from a free-kick Watford could not get away.
Femenia had to go off after half an hour after ending up on the turf in front of the dugouts and - suddenly - Millwall twice went close to an equaliser in a matter of seconds. Bachmann dived to parry Mitchell's shot before Bennett's deflected effort clipped the bar.
The half ended with Millwall on top but still trailing, and the second soon saw Bradshaw clip an effort just over the Hornets' goal.
Bennett was back within seconds for another shot that was too high and the Hornets were caught out again when Bradshaw got in behind Francisco Sierralta but Scott Malone was unable to profit from the cross that followed.
Watford made a double change on the hour, with Nathaniel Chalobah and Andre Gray coming on, and felt they should have been awarded another penalty straight away when Sarr went down under Mahlon Romeo's nudge in the back.
Bachmann had to claw away Bradshaw's deflected header from yet another free-kick, while Gray saw a glancing header saved at the other end.
Watford looked nervous but held out to celebrate referee Dean Whitestone blowing the final whistle.
'Watford efficient - and Sarr the dangerman'
Sky Sports' Paul Merson:
"Watford were efficient. I've watched them a few times recently and they've chipped away at the games and it's been enough. Fair play. They settled the nerves early with the penalty - Sarr is their real dangerman - and as soon as they got in front, you didn't see them getting pegged back.
"They're very strong, very solid but they'll need a lot more next season. Will Sarr do it next season? He dipped in the Premier League last time but he's a handful. The big advantage he'll have is that teams will come out and have a right go and he is lightning; there's going to be a lot of grass for him to run into.
"He gets his head up, he's a good player and I think he's one who'll find it easier in the Premier League. But he's going to need players around him to give him the ball."
What the managers said...
Watford's Xisco Munoz: "I want to say thank you to everyone for their support because today is the day for our fans, who have given us a lot of support on social media. Now is the moment for us to enjoy because we worked very hard to go up. I miss the fans. I was a little sad when we finished the game because if our fans had been at the stadium it would have been an amazing moment.
"They can enjoy at home because we are in the Premier League now."
"I have been away from my kids for six months and it is very hard sometimes to be without the family. For me it is very nice when I can go back home. I finished the game immediately and spoke to them. My daughter always asks me why her father never picks her up from school, so today I am very happy because although I have missed spending time with them I have a big dream in my life."
Millwall's Gary Rowett: "We gave a silly penalty away. Billy has got to learn not to be so rash, but he is a terrific young player with a bright future. For about 15 minutes after that it looked as though we felt a bit sorry for ourselves, but we worked hard to try to get back into the game. After the first 20 minutes I thought we were excellent and we were probably the better side from thereon. Just that little bit of quality was missing.
"Good luck to Watford, they have got Premier League quality players and deserve to go up, and sometimes when you lose a game you have to look at yourselves honestly, but I don't think we could have done much more than we did."