Jack Lester broke the deadlock early in the second half and, when Jamie Ward added a second on the hour, Mansfield were facing yet another defeat.
The situation is becoming increasingly bleak for The Stags who are rooted to the bottom of League Two and, although they were marginally the better side in the first half, they did not force goalkeeper Barry Roche into a serious save throughout the 90 minutes.
Chesterfield also struggled to make their former goalkeeper Carl Muggleton work on a cold and grey afternoon and there was little in the way of goalmouth action to warm a 6,000 plus crowd before half-time.
Nathan Arnold might have done better when Simon Brown sent him in on the Chesterfield left in the 15th minute but a poor first touch allowed the ball to get away from him.
At the other end an excellent block from Jake Buxton prevented Ward getting in a clear shot at goal.
But the course of the match changed in the space of five minutes early in the second half when first Michael Boulding sent a low shot just the wrong side of Roche's left-hand post and then Lester opened the scoring.
Mansfield lost possession deep in the Chesterfield half and Phil Picken struck a 50-yard ball downfield for Lester to get away from the two central defenders and fire low past the advancing Muggleton for his 16th goal of the season.
It was an excellent finish and was matched by Ward's decisive second goal which came eight minutes later.
Jamie Winter picked out Ward with a cross-field ball, which the striker controlled first time and then went past Johnny Mullins before firing low from the edge of the area and inside Muggleton's right-hand post.
That was the end of the game as a contest and, although Mansfield huffed and puffed, they could not seriously threaten a Chesterfield team who were reduced to ten men for the last 15 minutes after Kevin Gray was shown a red card when he went through the back of Brown.
Matt Hamshaw saw a right-foot shot go narrowly wide but Mansfield trudged off the field facing a bleak battle for survival in the second half of the season while Chesterfield can look forward to better times after ending their disappointing form at Saltergate.