Garry Thompson struck two late goals as Wycombe Wanderers booked their place in the Checkatrade Trophy Third Round with a battling victory at The Den.
The Chairboys, who grabbed a dramatic injury-time FA Cup winner in SE16 virtually a year ago to the day, broke the deadlock with a 50th-minute penalty from former Lion Adebayo Akinfenwa, only for David Worrall to level three minutes later with a low cross-shot that squirmed in at the near post.
Fred Onyedinma then spurned a glorious opportunity to edge The Lions ahead, but saw his 71st-minute penalty saved by Jamal Blackman. As the game headed towards a sudden death penalty shoot-out, Thompson took centre stage, lashing home an 84th-minute second before a close-range tap in on 90 minutes sealed victory for the League Two side.
Millwall boss Neil Harris handed a second start of the season to Canadian prospect Kris Twardek, one of seven players in the starting XI to have progressed through the Academy ranks at some stage.
Territorially The Lions certainly dominated the opening half of Wednesday night's match, but were unable to make their possession count in front of goal.
Ben Thompson, who had scored his first senior goal in that FA Cup tie, went close to opening the account with a thumping 25-yarder that zipped narrowly off target, but then the half virtually developed into a personal battle between Onyedinma and the Wycombe rearguard, who thwarted his every attempt to get on the score-sheet.
Thompson did well to slip the ball through to Millwall's No.10 on 26 minutes only for a defender to block the striker's goal-bound shot. Onyedinma was nearly played in again, this time by Jimmy Abdou, only for former Lions youngster Will De Havilland to step in and intercept.
Onyedinma was clean through on goal 10 minutes before the break, but the goalkeeper’s presence proved too much for the Lions' attacker and his heavy touch went out for a goal kick.
Wycombe introduced the man-mountain that is Akinfenwa at the start of the second-half, and the powerfully built striker certainly made a big impact.
Within five minutes of his introduction Akinfenwa was sweeping home the opening goal with a calmly converted spot-kick after Sid Nelson had been penalised for a foul on Stephen McGinn.
However The Chairboys' celebrations were short-lived and just three minutes later Worrall broke down the right, looked up and whipped in an angled cross-cum-shot that caught Blackman unawares and the ball squeezed into the back of the net between the keeper and his near post.
There were howls of disapproval from the home fans - and a look of total disbelief from Onyedinma - as he appeared to be wrestled to the ground after breaking into the penalty area with a barnstorming run on 68 minutes.
Onyedinma was involved again just seconds later, flicking the ball into the path of Twardek, whose close range shot was blocked for a corner as The Lions began to build up a head of steam.
A flurry of strong challenges on the lively Onyedinma went unsighted by the officials until the 71st-minute when the frontman was sent sprawling after he weaved his way into the penalty area following a neat interchange with sub Harry Smith.
Holding onto the ball despite the plea of sub Smith to take the resulting spot-kick, Onyedinma stepped up but was yet again thwarted by the visitors, with Blackman throwing himself low to his left to push away the penalty at full stretch.
Wycombe sub Scott Kashket replaced McGinn on 75 minutes and he had an immediate effect, pulling the ball back to Akinfenwa who screwed his left footed shot wide with the goal gaping.
Thompson thumped home the first of his brace on 84 minutes past Tom King after excellent work by Akinfenwa.
Twardek went close to grabbing an equaliser in the 89th-minute minute only to see his firm header drop inches off target.
With Millwall pushing on in search of a late leveller, Wycombe took full advantage and put the tie beyond reach in injury-time when Thompson tapped home from two yards after good work from Kashket.
King, Cummings, Martin, Craig, Worrall, Thompson (Butcher, 66mins), Onyedinma, Nelson (Ferguson, 87mins), O'Brien (Smith, 65mins), Abdou, Twardek. Subs not used: Archer, Williams, Mbulu, Pavey.
Blackman, Jombati, McGinn (Kashket, 74mins), Thompson, Bean, Hayes (Akinfenwa, 46mins), Wood, Harriman, Rowe, Muller (Pierre, 58mins), De Havilland. Subs not used: Richardson, Jacobson.
De Havilland (82mins)
Akinfenwa (50mins, pen), Thompson (84, 90mins)
1,619 (away 88)