Justin Edinburgh tasted victory in his first game in the dugout as Gillingham earned a 2-1 win at the ABAX Stadium with skipper Jack Payne's goal against his former club counting for nothing.
Darren Ferguson made one switch to his starting line-up for the visit of Gillingham with ex-Gills midfielder Jack Payne replacing Jermaine Anderson in the middle of the park. Gabi Zakuani was named on the substitutes bench after helping his country DR Congo to third place in African Cup of Nations.
Marcus Maddison had the first effort of the evening, but his ambitious left-foot drive cleared the crossbar. Jonny Egan was cautioned for a high challenge on Joe Newell eight minutes into proceedings.
On 10 minutes, Posh somehow contrived to miss a simple chance to break the deadlock as a low James' cross was blasted onto the bar from two yards by Shaun Brisley.
Washington scuffed a shot across the penalty box after an excellent cut-back from James as official Mr Adcock took credit for an superb advantage, ignoring the blatant handball against Garmston.
Washington then picked out James at the back post as the chances continued to flow, but the former Hartlepool man couldn't convert as the ball bobbled across the six-yard box.
On 29 minutes, the visitors took the lead with their first attack of the game as Bradley DACK volleyed home from just inside the box past the despairing dive of Scott Loach. Incredibly the visitors had the lead completely against the run of play.
Newell fired wide of the near post moments after the re-start as Posh tried to bounce back from the obvious setback of falling behind in a game they had dominated to date.
Posh had appeals for a penalty kick turned down on 34 minutes as a Smith cross appeared to strike an arm, but nothing was given by either referee Mr Adcock or his assistant.
Bostwick was cautioned for a foul after a poor pass from Smith was seized upon by Marquis. Burgess was superbly denied by Nelson as the game moved into first half stoppage time.
From the resulting corner, Burgess was again denied by Nelson, this time his glancing header was smothered at the near post.
Posh made a change at the break with Aaron Mclean replacing Washington in attack.
Marquis scooped a shot over the bar after a tricky run from Dack saw him burst past two poor challenges. On 55 minutes, Posh drew themselves level as skipper Jack PAYNE turned and fired low into the corner against his former club after excellent approach play from Luke James against all the odds down the right.
Jermaine McGlashan replaced Galbraith just after the hour for the visitors. Maddison sent a speculative volley wide of the target with his right-foot on 67 minutes.
Cody McDonald volleyed a corner kick inches wide of the target on 70 minutes after Brisley had sliced a long range effort from Dack behind for the set piece.
Maddison dragged a shot wide on 76 minutes after wriggling into space as Posh pressed for a second goal.
However on 79 minutes, the visitors got themselves back in front as MARQUIS fired home past Loach at his near post, another goal out of completely nothing for the visitors.
Posh introduced Erhun Oztumer for Brisley on 84 minutes in an attempt to get back into the game.
Posh: Loach, Smith, Brisley (sub Oztumer 84min), Bostwick, Payne, James, Washington (sub Mclean 46min), Newell, Burgess, Maddison, Beautyman. Unused subs: Henry, Ntlhe, J. Anderson, Zakuani, H. Anderson.
Gillingham: Nelson, Garmston (sub Martin 75min), Egan, Ehmer, Loft, Hessenthaler, Marquis (sub Norris 81min), McDonald, Galbraith (sub McGlashan 61min), Dack, Hoyte. Unused subs: Morris, Legge, German, Dickenson.
Attendance: 4,449 (245 away fans)
Half-time: Peterborough United 0-1 Gillingham