The Saddlers led 3-1 at one stage with goals from Steve Corica and Junior (2) butJohnson hit back with his three goals and also set up a goal for Dougie Freedman.
Johnson made a triumphant return to the West Midlands with another matchwinningperformance. Johnson, who followed manager Trevor Francis from Birmingham City, was ina class of his own as he demonstrated with his one-man show.
Walsall were left kicking themselves as Johnson snatched a last-minute winner.
The Saddlers made a blistering start with two goals in the opening five minutes. Coricascored after two minutes at the third attempt when the Palace defence was caughtnapping.
Junior scored the second with a left footed shot and only some brilliant saves by AlexKolinko prevented Walsall building up an even more conclusive lead.
On four occasions he saved brilliantly from the blond-haired Junior and his inspirationhelped Palace to get back into contention when Johnson scored with an angled shotfrom the left.
Walsall, who tossed away a 3-1 lead for the second successive home game, notched athird goal in the 53rd minute when Junior finally beat Kolinko.
The Eagles suddenly found their wings when Johnson hit a 58th minute volley at JamesWalker, which rebounded to Freedman to notch a good goal.
Johnson equalised with a flashing header from Julian Gray's corner in the 64th minute andthen scored his winner in the fourth minute of injury time