Alex Nicholls had given Walsall a deserved first-half lead but Lester, who has been out with a broken arm, pounced in the 90th minute to earn Chesterfield only their third point in 12 games.
Chesterfield almost scored after 35 seconds when Craig Westcarr’s low shot was turned behind by Jimmy Walker but Walsall settled and exposed the home side’s defensive frailities in the 29th minute.
Anton Peterlin’s ball in the from the left found Nicholls completely unmarked and the striker’s shot from 18 yards found the bottom corner, although Tommy Lee got a hand to it.
Chesterfield had been poor in the first half but they put the Walsall defence under pressure in the second and almost equalised when Lester got in but was forced wide by Walker, who then saw a Danny Whitaker shot clip the top of his crossbar in the 78th minute.
The visitors had defended well but they cracked in the 90th minute when a deflected shot fell to Lester who turned and fired low past Walker to rescue a point Chesterfield’s second-half pressure deserved.