Two substitutes made their mark in the 1-1 draw at Prenton Park.
Teenager Iain Hume joined the action for Tranmere in the 51st minute and with his second touch just two minutes later powered home a header from a Gareth Roberts free- kick.
It was tough on Peterborough at this stage. they had been much the better side but twice were denied by the woodwork.
Andy Clarke hit a post with an excellent left-foot shot from 20 yards in the eighth minute and then headed on to the angle from a cross by Gareth Jelleyman in the 21st minute.
The frustration showed for Posh after they went behind. Simon Rea who had been excellent in central defence was booked for unsporting behaviour and within minutes Jimmy Bullard and Graham Hyde were both cautioned for dissent.
Tranmere found themselves down to ten men in the 82nd minute.
Skipper Graham Allen, after being booked in the 63rd minute for a foul, was yellow carded a second time and sent off for taking a dive in the penalty area.
United took full advantage. David Farrell went on as an 85th minute replacement for Jelleyman and two minutes later powered forward and beat two defenders before putting a fine angled left-foot shot past John Achterberg into the roof of the net.
Posh manager Barry Fry said: "It would have been an injustice had we lost the game. We were by far the better side in the first half and should have gone in at least two goals up." Tranmere manager Ray Mathias said: "There was no excuse for the sending off. It was stupid. It made it hard for us and it cost us but a draw was probably a fair result.
"We needed a kick-start at half time. We were lacklustre and gave the ball away too easily in the first 45 minutes."