A 97th-minute penalty by Crawley Town, robbed Walsall boss Brian Dutton of his first win in eight attempts since taking charge.
Emmanuel Osadebe gave the Saddlers a lead early in the second half with his first goal for the club.
Nichols scored in the seventh minute of stoppage time after goalkeeper Liam Roberts flattened Ryan Tunnicliffe.
Crawley places 10th in the tables