Ryan Harley had a penalty saved but Craig Noone's 61st minute goal appeared to have ended Brighton's six-game winless streak.
However, Simpson's rasping 81st-minute drive flew into the bottom corner to give Kenny Jackett's side a point.
The Lions had plenty of efforts early on but Harley squandered the first half's best chance from the penalty spot when David Forde saved his tame effort.
Noone was a constant menace for the visitors and he exposed Millwall's static defending, running in unchallenged to volley home Craig Mackail-Smith's cross.
The Seagulls looked to close the game out but Simpson's speculative effort caught Casper Ankergren by surprise to level matters nine minutes from time.
The hosts had won just once all year but they sensed blood when Brighton arrived at The New Den having lost four of their last six matches.
The visitors were handed a golden opportunity to take the lead with 18 minutes gone as referee Neil Swarbrick controversially awarded them a penalty when striker Mackail-Smith went to ground after being pressured by Darren Ward.
Swarbrick appeared unwilling to give the spot-kick but his assistant, who was near to the play, flagged for a foul and that presented Harley with a chance to break the deadlock.
However he looked indecisive as he ran up and his weak effort was parried by Forde before the stopper scrambled the loose ball away for a corner.
That save inspired Millwall to lay siege to Ankergren's goal. Simpson's vicious volley went narrowly over before the Lions missed a hat-trick of good chances.
Paul Robinson's goalbound effort was cleared off the line and Tamika Mkandawire could not keep the rebound down before Liam Trotter headed Brian Howard's cross wide from close range.
Noone had been creating problems for Millwall during the first period and that prompted Jackett to replace Scott Barron with Alan Dunne at half-time.
But nobody picked up the winger as he fired Gus Poyet's men ahead. Mackail-Smith's original cross came straight back to him but the Scottish international whipped the ball back in and Noone ran across the static Mkandawire before volleying home.
The hosts have won just once all season and seemed to be heading for another defeat as the visitors controlled possession - until Simpson collected a loose pass and unleashed a thunderbolt from 30 yards.
His speculative effort had power and pace but Ankergren was slow to react and the ball whistled into the bottom corner to ensure the game ended level.