Posh suffered a 2-1 defeat on the final day of the campaign as Rotherham United came from behind to secure all three points.
If anything, the final game of the season pretty much encapsulated the rest of the campaign. Posh started the game brightly, opening the scoring inside 60 seconds, missed a few chances and then gifted two goals to the opposition.
The Millers were ruthless in taking advantage as Sam Nombe and Cameron Humphreys got themselves on the score-sheet after Abraham Odoh's early effort.
ODOH's goal came inside the opening minute without the home side touching the ball, with the former Harrogate man cutting inside and firing home with his right-foot.
Gustav Lindgren was denied by Dillon Phillips after racing onto a Tayo Edun pass, while Edun himself was also kept out by the keeper after a slick Posh move.
Will Blackmore was beaten on 35 minutes, the initial effort from Powell struck the underside of the crossbar and was probably over the line before NOMBE beat two Posh players to the bouncing ball to head home.
Blackmore kept out a deflected Wilks effort on 42 minutes before Conn-Clarke sent a free-kick inches wide of the near post in stoppage time.
Blackmore did well to deny Nombe three minutes into the second half before Rotherham got their noses in front as HUMPHREYS headed home a corner.
Posh handed debuts to Joe Andrews and Andre Changunda with both catching the eye but aside from a long-range De Havilland strike, it looked like Posh wouldn't come closer.
Posh did hit the post on 88 minutes as Edun let fly from distance after Andrews saw his shot blocked.
Rotherham: Phillips, Rafferty, Kelly, Powell, Nombe, Wilks (sub Kayode 86min), Sibley, Gore (sub McWilliams 61min), Odoffin, Humphreys, Mpanzu (sub Holmes 61min). Unused subs: Clarke-Harris, Hugill, Hull, Richardson.
Posh: Blackmore, Johnston, Katongo (sub Andrews 79min), Nevett, Mills (sub Changunda 66min), Edun, De Havilland, O'Brien-Brady, Odoh, Lindgren (sub Jones 60min), Conn-Clarke (sub Dornelly 60min). Unused subs: Steer. Mendonca, Shofowoke.
Half-time: Rotherham United 1-1 Peterborough United
Full-time: Rotherham United 2-1 Peterborough United
Attendance: 9,994 (1,240 Posh fans)
Referee: M. Barlow