It looked like Wayne Rooney’s Rams were going to pinch the points after taking the lead through young Academy graduate Jack Stretton in the 76th minute.
But, in what turned out to be a tale of three substitutes, Posh replacements Harrison Burrows and Siriki Dembele turned the game in its head. Burrows tapped him Joe Ward’s cross for a 91st-minute equaliser before another Ward cross led to Dembele converting from 12 yards nine minutes into 11 minutes of added time.
It’s a first Championship win of the season for Posh who had suffered a shocking first week of the season. It’s a result, and a much improved performance, which should give the players, and the fans, a massive confidence boost.
Not that Derby will be one of the second tier strongmen. Indeed cynics would suggest this was an early-season six-point relegation battle. A shootout for 21st place maybe.
There was a boost for Posh pre-match with the suprise inclusion of striker Jack Marriott against his former club, but also a blow as Sammie Szmodics sat this one out following the birth of his first child. Marriott was chasing his first goal in 11 months.
Oliver Norburn was handed his Posh debut in the centre of midfield in a 3-4-1-2 line-up. Joe Randall was deployed behind Marriott and Clarke-Harris with Nathan Thompson in the middle of a back three.
Posh made a bright start. They won headers, they snapped into tackles before the game settled into a cagey affair with neither side creating any first-half chances.
The best fell to Marriott after a fine exchange of passes between Randall, Butler and Jorge Grant. Randall’s cross teed up Marriott 12 yards from goal, but instead of the first time shot the pass demanded the striker took a heavy touch and the chance was gone.
Butler was enjoying the freedom of the left side without finding a decent cross which was a shame as Clarke-Harris looked to have the beating of the Derby centre-backs in the air.
Clarke-Harris almost gave Posh the lead 10 minutes before the break with a speculative 25 yard strike which Derby goalkeeper Kelle Roos made a real hash of with the ball spilling behind him but not far enough to reach the net.
Derby enjoyed spells of possession, but they lacked penetration and a decent set-piece taker. Posh keeper Christy Pym had just one soft shot from Graeme Shinnie to deal with in a 45 minutes in which the home side failed to win a single corner.
The second-half was an altogether livelier affair. Jorge Grant tested Roos in the first minute from 20 yards before powerful striker Colin Kazim-Richards was carried for the visitors. If anything it was a boost for Derby who suddenly had more energy and legs.
Former Manchester United player Ravel Morrison, who was a massive disappointment, wasted a great breakaway with an errant pass and was then denied a possible goal by a superb Frankie Kent block.
A left-wing cross just eluded Stratton, but the Academy graduate wasn’t to be denied for long as a glaring Posh weakness at defending corners was exposed.
Derby won the header at the far post, Kent’s clearing header didn’t leave the penalty area and Stratton swivelled to fire home superbly on the half volley.
Dembele had made his long-awaited entrance for Marriott almost 20 minutes earlier. He had immediately promised much by freeing Clarke-Harris in the area for a shot that was blocked.
Clarke-Harris played a small part in the equaliser by flicking on a long Butler pass leaving Ward to land a precision cross at the feet of Burrows.
And although Ward’s 99th minute cross was half cleared Dembele was on hand to steer the ball into the corner of the net to complete the sort of come-from-behind win that became commonplace in League One last term.
Cue bedlam in the Posh areas of a 10,000-plus crowd.
Posh: Christy Pym, Dan Butler, Mark Beevers (sub Harrison Burrows, 84 mins), Frankie Kent, Nathan Thompson, Joe Ward, Jorge Grant, Oliver Norburn, Joel Randall (sub Josh Knight, 73 mins), Jack Marriott (sub Siriki Dembele, 63 mins), Jonson-Clarke-Harris.
Unused subs: Dai Cornell, Ronnie Edwards, Kwame Poku, Joe Tomlinson
Derby: Kelle Roos, Nathan Byrne, Richard Stearman, Curtis Davies, Craig Forsyth, Graeme Shinnie, Max Bird, Festy Ebosele (sub Louis Sibley, 58 mins), Tom Lawrence, Ravel Morrison (sub , Kamil Jozwiak, 84 mins), Colin Kazim-Richards (sub Jack Stretton, 51 mins).
Unused subs: Ryan Allsop, Jordan Brown, Dylan Williams, Louie Watson,.
Goals: Posh – Burrows (90 + 1 mins), Dembele (90 + 9 mins).
Derby – Stratton (76 mins)
Cautions: Posh – Beevers (foul), Thompson (foul), Butler (foul), Dembele (over-zealous celebration).
Derby – Forsyth (foul), Sibley (deliberate handball).
Referee: David Webb 7.
Attendance: 10,177 (1,619 Derby)