The visitors, who started the game 89 places below Posh in the football ladder, won 2-1, but in truth a far greater margin of victory wouldn’t have flattered them.
Chorley fell behind to Jack Taylor’s second minute strike, but struck the woodwork twice before the break, scored twice in three second-half minutes and saw a late penalty saved by Posh ‘keeper Christy Pym.
Posh were collectively woeful. They passed without any tempo or accuracy, defended appallingly and created nothing until the 95th minute when Siriki Dembele saw his close range shot smothered by Chorley ‘keeper Matt Urwin. An equaliser would have been a travesty.
In contrast Chorley were excellent. They set up superbly. Never gave Posh an easy pass and snapped into tackles whenever the ball got into their half.
Peterborough United should make a couple of changes to their starting line-up for the visit of Sheffield Wednesday
Posh chairman Darragh MacAnthony: 'Put the hard yards in or we'll get beaten'
Peterborough United manager Grant McCann explains why it’s difficult finding a replacement for Sammie Szmodics.
Poku is too important to be risked by Peterborough United, plus injury updates on Butler, Norburn and Fernandez
Peterborough United talking points: 'The diminishing value of Ronnie Edwards, a manager more impressed by hard work and performance than reputation and Bali Mumba's behaviour'
As expected Posh fielded a strong side with Mo Eisa replacing injured top scorer Jonson Clarke-Harris in the only change from the League One win over Plymouth earlier in the week.
Top of manager Darren Ferguson’s wish list would have been an early goal and all seemed well when Taylor lashed a half volley into the roof of the net following a half-cleared corner in the second minute.
But any thoughts of a landslide victory soon disappeared as Chorley’s well organised defence was accompanied by some slick counter-attacking. They also had the option of the long ball to powerful striker Harry Cardwell who gave the three Posh centre-backs a torrid time.
The visitors’ threat was a while coming though as Eisa forced a flying save from Chorley ‘keeper Matt Urwin and Siriki Dembele whistled a free kick just past a post.
But as Posh became laboured in possession - they clearly missed the out ball provided by Clarke-Harris - Chorley’s confidence grew and they were most unlucky not to equalise before the break.
Cardwell blasted against the post from the edge of the area in the 28th minute with Pym rooted to the spot and in added time Elliott Newby’s thrilling run from inside his own half ended with a shot against the post. Cardwell had a great chance from the rebound but scuffed his shot wide.
Posh created little with Sammie Szmodics playing Eisa through, but the striker cut inside onto his weaker left foot and dribbled his shot straight to Chorley ‘keeper Matt Urwin.
Posh would have been grateful to reach half-time in front, but there was no improvement after the break as Chorley continued to look like the League One side. They were quicker in deed, brighter in possession and defended with power, strength and organisation.
It still took some dozy defending to gift Chorley their equaliser as centre-backs Frankie Kent and Mark Beevers left a left-wing cross enabling Connor Hall to nip in and fire home in the 61st mninute.
And two minutes later a well worked corner led to Lewis Baines drilling home the winning goal from 10 yards.
The Posh reponse was pitiful. They changed shape to a flat back four, but never looked like penetrating and indeed Chorley should have wrapped the game up 14 minutes from time.
Mark Beevers was caught out by a deep corner and flattened Cardwell for an obvious penalty. Cardwell took it and hammered it, but Pym saved well.
Not even such a giant reprieve could galvanise Posh who resorted to lumping the ball forward to Beevers who was acting as an emergency centre-forward.
They broke through once when Taylor and Dembele combine, but Urwin made a smart save to ensure long and loud celebrations followed at the final whistle.
Posh: Christy Pym, Dan Butler (sub Niall Mason, 65 mins), Mark Beevers, Frankie Kent, Frazer Blake-Tracy, Ryan Broom (sub Idris Kanu, 69 mins), Reece Brown, Jack Taylor, Siriki Dembele, Sammie Szmodics (sub Flynn Clarke, 57 mins), Mo Eisa.
Unused subs: Dan Gyollai, Ronnie Edwards, Louis Reed, Ethan Hamilton
Chorley: Matt Urwin, Arlen Birch, Harvey Smith, Andy Halls, Scott Leather, Lewis Baines, Mike Calveley, Harry Cardwell, Elliott Newby, Ollie Shenton (sub Adam Henly, 90 + 3 mins), Connor Hall (sub Will Tomlinson, 86 mins).
Unused subs: Lewis Reilly, Oliver Putnam, Tyler Garratt, Malakai Mckenzie, Sewa Marah.
Goals: Posh - Taylor (2 mins).
Chorley - Hall (61 mins), Baines (63 mins).
Cautions: Posh - Taylor (foul), Brown (foul).
Chorley - Shenton (foul).
Referee: Darren Drysdale 7