Mark Allott, who had not scored this season, sparked off Chesterfield's revival with two goals before Wayne Allison grabbed a 77th minute winner.
The vital turning point occurred at half time when Chesterfield's manager, Roy McFarland, drafted in Allison and Colin Larkin.
Walsall caved in against the attacking pressure applied by the pair and Chesterfield recorded their third away win of the season.
Dismal Walsall have now only won one of their last seven games.
There was little excuse for Walsall's second-half collapse although they were forced to take a big gamble when they had to draft in their teenage goalkeeper Rene Gilmartin for his Football League debut after Andy Oakes was injured in the warm-up.
Chesterfield surprisingly failed to pressurise the youngster and Walsall capitalised upon the situation to gain the upper hand to produce some convincing attacking play.
The breakthrough came in the 14th minute when the Saddlers steamed ahead when Jorge Leitao was on the mark to finish off a low right-wing cross from Matty Fryatt.
Chesterfield took some time to settle but eventually began to string together some good attacks in which Paul Hall was to the fore against his former club.
But their determination to get back on level terms back-fired when Walsall consolidated their lead in the 35th minute.
Gilmartin began the move when he put Daniel Fox in possession but it was Fryatt who created the vital pass when he turned Reuben Hazell before sliding a pass inside Gareth Davies.
Mark Wright ran through and, as he closed in on Chesterfield's goal, slid his shot goalwards and Steve Blatherwick's desperate bid to clear on the line only succeed in lashing the ball into the net.
The second half was a transformation with hesitant Walsall slumping badly.
Allott was on the mark for the first time with a header following a Kevan Hurst corner in the 55th minute.
Walsall hit the panic button and it was hardly a surprise when Allott produced his second goal with a diving header following another Hurst corner.
Chesterfield were now flying against a Walsall side in total disarray and Allison was on hand to apply the finishing touch to a left-wing cross from skipper Shane Nicholson.