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Millwall have suffered just one defeat at home this season and the 2004 FA Cup finalists deservedly took the lead through Lewis Grabban four minutes after the break.
But Commons, making his first start for Derby since October, hit back almost immediately after Millwall failed to clear their lines.
Only 13 places separated the two sides at kick-off but Derby's Chris Porter forced the first save of the afternoon with a tame header that was easily collected by David Forde, while Millwall's Dave Martin saw his goal-bound shot blocked.
The best chance of the opening 45 minutes fell to Grabban who was released by Chris Hackett but a poor first touch forced the former Crystal Palace man on to his weaker left foot and he shot wide.
Commons found himself unmarked on the left side of the box as Derby pressed in first-half stoppage time and the Scotland international's deflected cross had to be headed off the line by Millwall captain Paul Robinson.
But it was The Lions who broke the deadlock just four minutes after half-time with the best move of the match.
Grabban's looping header capped a break, which saw Jack Smith, Steve Morison and Hackett link up, with the latter's cross finding Grabban 18 yards out.
But that lead last only three minutes when Millwall were unable to clear Dean Moxey's cross from the right and Commons equalised with a fierce snap-shot.
Martin came within a whisker of putting the home side back in the lead when his deflected 35-yard strike went narrowly wide with Stephen Bywater beaten.
Kenny Jackett's side continued to press for the winner and Morison's 77th-minute header was cleared off the line by Moxey, while Derby substitute Steve Davies saw his free-kick go narrowly over.