Boro had to respond to Nottingham Forest’s big win at Blackpool earlier in the day to keep their EFL Championship playoff push alive thanks to goals from Marcus Tavernier, Folarin Balogun, Josh Coburn and Duncan Watmore.
Peterborough were looking to build on their first win under Grant McCann after the victory over Queens Park Rangers before the international break but hardly forced a save out of Joe Lumley all afternoon, to cement themselves to the foot of the division.
Story of the Game
The game started at a good tempo, with the hosts having a couple of half-chances, including a low Jonson Clarke-Harris drive which was dragged past the far post.
Emmanuel Fernandez was an imposing figure at the back and looked comfortable in possession on his Posh debut in the early stages.
The visitors grew into the game and a pair of chances, firstly through Matt Crooks and then Aaron Connolly saw a header spin past the post and a deflected shot into the hands of Dai Cornell.
Frankie Kent intercepted the latest foray forward from Boro and set Sammie Szmodics on his way.
Lumley beat the former Colchester United midfielder to the ball but his clearance fell to the feet of Jack Marriot and the striker looked to curl one into the empty net with his weaker, only for Paddy McNair to clear behind for a corner.
Minutes later, Peterborough were left to rue their missed, speculative effort as Middlesbrough took the lead in stunning style.
Marcus Tavernier found himself in acres of space on the edge of the box from a corner and his effort curled away from a static Cornell to give the playoff chasers a deserved lead.
For all their domination and possession, Middlesbrough didn’t create a clear opening again before half time, and almost paid for that.
The hosts responded well to being a goal down and a few chances started to fall their way.
The busy Szmodics was found delicately by Clarke-Harris but he tried to take the ball past Lumley in the away goal instead of lifting it over him.
Then last season’s League One top scorer had two chances of his own from a corner – his initial header was blocked before the rebound dropped to him inside the six-yard area, but Lumley saved well.
This snapped Boro back into life and should have doubled their lead right before the interval as Connolly slipped by Kent and fizzed it across the box for but captain Howson couldn’t get on the end of it.
Middlesbrough doubled their lead four minutes after the break after a mix-up between Cornell and young Fernandez.
Josh Knight missed his header meaning the debuting defender let Howson’s cross bounce past him straight into the path of Balogun, who side-footed home inside the area.
Posh responded well again, and half-time substitute Ricky Jade-Jones started to cause problems, and a Marriott volley went harmlessly wide of the post.
Peterborough made a couple of changes and made a switch in system, and gained some control on the game, but couldn’t create a chance of note.
And the game was put to bed with seven minutes remaining.
Substitute Duncan Watmore put his head down and took on a tiring Jeando Fuchs and burst into the box, squaring it for fellow replacement Josh Coburn to deflect it home.
And Watmore turned scorer with a sublime finish in the 90th minute, dispossessing Fuchs in the middle of the Posh half and sent the away supporters into bedlam.
That was how it finished, with Middlesbrough leaping into fifth place while Posh sunk to the bottom.
Cornell; Knight, Fernandez, Kent; Ward, Fuchs, Taylor, Burrows; Szmodics; Marriott, Clarke-Harris.
Lumley; Dijksteel, McNair, Fry; Jones, Crooks, Howson, Tavernier, Bola; Connolly, Balogun.