Lester struck twice in the space of five minutes and turned the game on its head before Jamie Ward added a late third to seal the win.
Chesterfield handed debuts to five summer recruits, while Barnet's new pair of Nicky Deverdics and Luke Medley had to settle for a place on the bench.
It was the visitors who looked the most comfortable early on with Ward going close on two occasions, bringing a fine save out of Lee Harrison in the 17th minute.
But a minute later the Bees went ahead in bizarre fashion when Ashley Carew's aimless free-kick from fully 50 yards caught debutant keeper Tommy Lee off his line and the ball sailed into the Chesterfield net.
Ward should have atoned for Lee's error within minutes but squandered a great chance from Paul Harsley's pass.
And twice before the interval, Lee Richardson's men threatened an equaliser, Harrison saving from his own skipper Ismail Yakubu before Lester saw a fine strike ruled out for pushing.
Lester was not to be denied in the 57th minute when he burst through on goal and calmly slotted past Harrison to grab a deserved equaliser.
And he showed even more poise to put the visitors ahead just after the hour-mark with a delicate chip over Harrison after Ward had fallen under a heavy challenge.
A stunned Barnet pushed forward attempting to get back on terms but Lee atoned for his earlier error with two fine saves to deny Adam Birchall and the lively Albert Adomah.
Any hopes of a point were extinguished, however, when Yakubu misjudged Alan Goodall's punt forward allowing Ward to run through and complete the win with a calm finish.