Willock bagged the only goal of the game in the 17th minute to earn Posh a rare away victory.
But results elsewhere ensured the three points were still not enough to keep Peterborough's slim survival hopes alive.
Play-off chasing Bournemouth came into the game looking to avoid a fifth successive defeat at their Fitness First Stadium home.
The Cherries could have found themselves in front as early as the first minute when defender Eddie Howe put a header across the face of the Peterborough goal from Brian Stock's corner.
The home side had another near miss three minutes later when leading goalscorer James Hayter volleyed over from Stock's centre.
On-loan Arsenal defender Frankie Simek then headed over from the resulting Stock corner before Stephen Purches put a 25-yard strike straight at goalkeeper Mark Tyler.
Peterborough had their first meaningful effort on goal when veteran striker Andy Clarke scuffed the ball into the arms of Neil Moss from the edge of the penalty area in the 16th minute.
Barry Fry's side took a surprise lead a minute later thanks to Willock's 12th league goal of the season.
Experienced midfielder and skipper Curtis Woodhouse outpaced Simek before crossing for the gangly Willock who picked out the top left-hand corner with a brilliant header.
Striker Hayter should have equalised in the 28th minute when he sliced over from six yards after Stock had beaten the Peterborough offside trap with a lofted through ball.
Stock came close with a shot that trickled just wide of the right-hand post five minutes into the second half.
Matthew Mills headed against the crossbar on an afternoon of frustration for Bournemouth.