Lee Gregory kept Millwall on course for an immediate return to the Championship.
The striker bagged his 27th goal of the season to set the Lions on their way to a 4-2 aggregate win over Bradford in the play-off semi-final.
Jamie Proctor made it 1-1 on the night just before half-time, but Bradford failed to stage a second-half revival.
Millwall boss Neil Harris, who could not save them from the drop last year, now has the chance to secure promotion in his first full season in charge.
The Lions' record goalscorer was in the side which beat Swindon at Wembley the last time they were promoted in 2010, and he will return to lead them out against Barnsley a week on Sunday.
The hosts almost got off to a dream start inside the first minute when Steve Morison's ball over the top sent Gregory through but he was denied by keeper Ben Williams.
A slip from Bantams skipper Stephen Darby then allowed Gregory to race forward before sliding the ball to Ben Thompson with the goal at his mercy.
But as the young Lions midfielder got his shot away, West Ham loanee Josh Cullen slid in to make a vital last-ditch block.
Bradford almost brought the game to life just after the half-hour mark when Tony McMahon swung in a corner which Nathan Clarke headed narrowly wide.
Clarke held his head in his hands, knowing how good a chance he had missed, and that point was rammed home moments later when Gregory fired Millwall ahead.
Morison skipped past two challenges as he darted into the box before squaring the ball to his strike partner who neatly slotted home from 18 yards for his 27th goal of the season.
Millwall almost went two up on the night, and four in total, when Joe Martin whipped in a free-kick but Williams tipped his effort over the crossbar.
And Bradford dragged themselves back into the tie a minute before half-time, Proctor tucking home Kyel Reid's cross at the second attempt after Jordan Archer saved his initial effort.
The Bantams still needed two goals to force extra time but, to the frustration of their fans, did not muster a shot on target after the break.
Instead, with a raucous Den in full voice, it was Millwall who came closest to scoring again with Jimmy Abdou volleying over and Bryon Webster heading wide.