The two teams had been beneficiaries of the abridged 19/20 season, pushing Posh out of the League One playoff picture thanks to the weighted PPG system. All but a handful of teams had voted to end that season early, including the Millers and Chairboys.
Furious Posh owner Darragh MacAnthony then branded this season 'Operation Vengeance' and they confirmed their promotion last week, pulling themselves up from three goals down.
It meant that Peterborough fans could relax going into this weekend, and many will be delighted by today's turn of fate .
Wycombe were all but relegated going into today's games, but their 3-0 win against Middlesbrough did at least pull them from the bottom of the table.
Rotherham were still in with a chance of survival - and were moments from a famous win against Cardiff that would have kept them up after Lewis Wing's early screamer.
However an 89th minute Marlon Pack goal to make it 1-1, coupled with a frenetic 3-3 draw at Pride Park, sent Paul Warne's side to the third tier.
Sheffield Wednesday are the third team to fall, with a six-point deduction ultimately proving decisive. Meanwhile today's rivals Derby stayed up thanks to two goals from Martyn Waghorn.
Posh will be joined in the Championship by League One winners Hull, and the winners of the playoffs. Peterborough's game tomorrow against Doncaster is largely irrelevant.