The young striker, still just 18, had already forced a fine save from Posh goalkeeper Paul Jones when he put the icing on the cake for the Lions with a scorcher for the third goal in the final minute.
Harry watched the ball over his shoulder and hit a full, dipping volley from the left corner of the box into the far corner.
"I've scored 10 senior goals now and that's the best," said Harry.
"I've had a few chances in recent weeks, hit the woodwork, seen goalkeepers make great saves, so it was great to get on the scoresheet again.
"The most important thing was the result and it was a great three points for us.
"That's two goals in the Championship now and I'll keep going to get as many as I can. It's a fantastic experience for me."
Ryan Mason was an unused substitute at London Road.
Staying in the Championship and it was a good night for Tom Carroll, who played 68 minutes as Derby managed to turn around an early deficit to beat in-form Blackpool 2-1 at Pride Park.
It wasn't all good news for our players on loan, however, as Dean Parrett picked up a red card for two bookable offences as Yeovil produced a surprise 1-0 win at MK Dons in League One.
Andros Townsend was outstanding for Birmingham City against Derby County - but team-mate Tom Carroll had the last laugh as the Rams came from 2-0 down to draw 2-2 at St Andrews.
Winger Andros' shot was turned home by Erik Huseklepp for the opening goal in the first half and the same player glanced his corner onto the crossbar.
Into the second half and Andros saw his shot cleared off the line by Gareth Roberts before he dashed 50 yards upfield on a breakaway and rolled the ball into the path of Marlon King, who drilled home for 2-0.
Undetered, Derby struck back through Steven Davies for 2-1 after 61 minutes and levelled through Theo Robinson five minutes later.
That's how it stayed as Andros played the full 90 minutes and Tom completed 85.
Staying in the Championship, Harry Kane released Andy Keogh to fire Millwall ahead against Reading at the New Den.
Reading levelled before the break and Harry missed a decent headed chance at 1-1 before the Royals grabbed maximum points through Adam Le Fondre. Harry played 83 minutes and Ryan Mason came on as a late substitute for the Lions.
In League One, Jon Obika and Dean Parrett both started as Yeovil completed a key 2-1 win against Brentford on Saturday.
Turning to the Premier League and Steven Caulker provided the pass for Gylfi Sigurdsson to strike home the first goal as Swansea beat Wigan 2-0 at the DW Stadium.
Steven Pienaar helped Everton grab a 1-1 draw at QPR and Kyle Naughton started as Norwich slipped to a 1-0 defeat at Stoke.
Kudus Oyenuga was an unused substitute as St Johnstone managed a goalless draw at Kilmarnock in the Scottish Premier League.
In Germany, injury ruled out Vedran Corluka taking his place as Bayer Leverkusen managed a fine win against Bayern Munich on Saturday. It may yet keep him out of Wednesday night's glamour tie in the Champions League as Bayer travel to the Nou Camp in the last 16, second leg, trailing Barcelona 3-1 from the first leg.