Full-time score from London Road
On an afternoon that was great entertainment for the fans but would have left both managers tearing their hair out at some of the defensive play, The Lions twice battled back from 2-0 down to 2-2 and then from a 4-2 deficit to 4-3 before a late goal from Jon Taylor - his second of the game - finally settled this pulsating affair.
The 1,400 travelling fans were in fine voice before the game even kicked off, drowning out the pre-match music and giving Harris a rousing reception as the manager took his place in the away dug-out.
There was an air of confidence in the Millwall camp following a run of four wins from the last six outings, but unfortunately it proved to be an opening 45 minutes of hard knocks for the south east Londoners.
The Lions suffered a setback with less than four minutes on the clock when midfielder Jimmy Abdou was forced off with a shoulder injury after a promising break from the halfway line was brought to a halt by a cynical foul from Marcus Maddison, who was unpunished for his offence.
On came Ben Thompson to take his place in centre midfield alongside Shaun Williams on what was his 20th birthday.
Just three minutes later the visitors created the first clear chance of the game when some deft footwork by Aiden O'Brien prised an opening for Steve Morison, the latter's shot hitting the target but lacking power and was comfortably collected by Ben Alnwick in the home goal.
O'Brien then picked out Onyedinma with a wonderful backheel on the right flank as The Lions continued to prise and probe the home defence. It took the linesman to come to Posh's rescue on 11 minutes when a superb threaded through ball from Williams picked out O'Brien, but the forward's perfectly timed run that had put him clear through on goal was halted by the referee's assistant, who raised his flag for offside.
The levels of frustration increased just four minutes later when Maddison twice went to ground theatrically in quick succession, although jumped up fit enough to be involved in both dead ball deliveries. The first he hit directly into the hands of Jordan Archer from 20 yards and the second, from wide on the right, was only half-cleared and when Taylor's shot rattled the post, Angol reacted first to nod the rebound home for the opening goal in the 15th-minute.
Matters went from bad to worse for Millwall just four minutes later when the diminutive Taylor was allowed to carry the ball forward and took aim from 20 yards with a thumping effort that crashed into the roof of the net with a shot that gave Archer no chance.
It was a double body-blow to The Lions, who tried in vain to work their way back into the match for the remainder of the half.
Lady Luck looked to possibly favouring Millwall with Peterborough having to make a substitution of their own after half an hour when keeper Alnwick feel awkwardly trying to collect a corner and was replaced by 18-year-old England youth prospect Dion-Curtis Henry.
Although not having to make any saves of note for the remainder of the half, the young stopper, whose huge physique certainly belied his tender years, was rooted to the spot as shots from Williams, Ed Upson and Onyedinma whistled past his goal in quick succession.
A sweeping move six minutes before the break nearly produced Millwall's best opening of the half when Edwards and Onyedinma combined on the right and worked the ball to Upson in a central position, but his first-time shot dropped narrowly wide of the upright.
However The Lions did get a foothold back in the game courtesy of centre-half Byron Webster's first ever goal in a Millwall shirt, directing a powerful header beyond Henry's reach from Williams' pinpoint corner delivery seven minutes into the second-half.
Fellow defender Tony Craig sent the travelling fans wild just three minutes later with his first goal since rejoining the club in the summer. Some fine combination play by Onyedinma and Edwards on the right ended with the latter's deep cross headed into the roof of the net by skipper Craig to bring the score back to 2-2.
Michael Bostwick denied the rampant Lions a third goal with an excellent block from Upson's goal-bound shot after Steve Morison had slipped the ball to his team-mate eight yards out. Unfortunately Upson injured himself in that move and gingerly hobbled off moments later to be replaced by Shane Ferguson, who slotted in on the left side of midfield.
With The Lions still readjusting to their second enforced change of the game, Posh powered in another two quick-fire goals to ease 4-2 ahead and seemingly put the result beyond doubt.
Sub Chris Forrester restored the home side's lead with a low drive on 68 minutes and a thumping finish from Conor Washington after an unfortunate slip by Webster looked to have secured the points for Peterborough with a fourth on 72 minutes.
But Millwall were right back in it again on 74 minutes when Beevers took advantage of some slack defending to head home another Williams' corner, becoming the third Lions defender to score on what was developing into a slightly surreal afternoon.
No sooner had they edged to within a goal of their hosts, than The Lions went 5-3 behind courtesy of Taylor's second - and Posh's fifth - of the game, another thumping finish after Maddison had flicked the ball into his path.
Such was the fragile nature of both team's defending that you always got the feeling there were going to be more goals in this game.
Incredibly there weren't, however, although Lions sub Gregory was denied by the keeper from close range, Beevers lashed narrowly over and Williams rattled the crossbar with a 25-yard piledriver having also sent another shot from distance inches over the bar.
Posh were denied a sixth goal in the third minute of injury-time when Maddison's fierce free-kick was clawed away at full stretch by Archer, who then denied Angol with his legs following an incredible passage of attacking play.
Unfortunately the damage had already been done and The Lions headed back down south licking their wounds after a first away defeat in six games.
Scorers: Angol (15mins), Taylor (19, 81mins), Forrester (68mins), Washington (72mins)
Millwall: Archer, Edwards, Craig, Beevers, Webster, Williams, Abdou (Thompson, 4mins (Gregory,75mins)), Upson (Ferguson, 61mins), Morison, O'Brien, Onyedinma. Subs not used: Forde, J.Martin, L.Martin, Marquis.
Bookings: Thompson (64mins), Williams (76mins), O'Brien (90+3mins)
Scorers: Webster (52mins), Craig (55mins), Beevers (74mins)