But the Lions dominated against Swindon to earn their place in the second tier of English football, something Jackett has never previously experienced as a manager.
"I have worked hard for it so I'm looking forward to working in the Championship for the first time," said Jackett, who has also managed Swansea and Watford.
"This is the best moment of my career, there is nothing close.
"It is great to finish it off this way. I'm really pleased for everyone at the club. We need to look forward quickly because we want to go into the next league and compete."
And 23-goal top scorer Steve Morison echoed the thoughts of his manager: "It is an absolute dream. We are going into the Championship next year and we are going to show people what we can do there as well."
Despite his disappointment at missing out on promotion, Swindon manager Danny Wilson congratulated the London side.
"Millwall were in this position last year so you say well done to them," said Wilson.
The Robins, who finished two places and three points behind Millwall in League One, had a superb opportunity to equalise in the second half.
The chance fell to 20-goal striker Charlie Austin when he latched on to Paul Robinson's misplaced header but Austin scuffed his shot well wide when one-on-one with the goalkeeper.
"If you break it all down they had the first half, we had the second but we missed a header from a corner and got a bobble when Charlie was through," explained Wilson.
"You could see it straight away - it just bobbled up on him. I'm gutted for him, he's taken it to heart but it's not down to him.
"When you get here you try to finish it off but unfortunately we haven't."
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