Former Chesterfield attacking stalwart Jason Lee unlocked his old club's determined defence within 90 seconds of joining the action as a late London Road substitute, setting up Peterborough's first victory in nine outings.
The lanky Lee nodded an 82nd minute cross by Matthew Gill right into the path of Andy Clarke and the veteran Posh top scorer shuffled his way past Kevin Dawson before popping in his ninth goal of the campaign from a couple of yards.
Clarke's effort was an immense relief for under-fire Posh boss Barry Fry who couldn't stop himself dashing on to the pitch for a dance of delight.
"Andy has been marvellous for us all season and scored a vital goal in a game played in what I found to be one of the most hostile atmospheres I have experienced in football," said Fry.
He added: "There is obviously a vendetta against me at present and it got through to he lads who were nervous at the start, but we improved as the game went on and thoroughly deserved to win." Chesterfield were restricted to a handful of shots all afternoon and the nearest was in the early stages when Chris Brandon fired past Mark Tyler's left-hand post from 20 yards.
Posh turned the screw after the break with Jimmy Bullard bringing a superb one-handed save from Carl Muggleton, although the goalkeeper was beaten by a curling 20-yarder from Posh's Helgi Danielsson, only for the ball to smack against the woodwork and fall to Bullard who blazed high and wide.
Chesterfield boss Dave Rushbury sympathised with his side's travelling fans, commenting: "I'm bitterly disappointed with what the forwards gave me.
"We hardly had a shot and it has to improve, it's as simple as that.
"We are still in the top ten because we are good at the back. Going forward we are absolutely useless."