But Commons secured a replay for the Championship side moments later when he latched on to a rare mistake in the Millwall back line.
With Derby struggling in the league and having won just once on the road all term, Millwall went straight on the attack.
They almost went ahead after 14 minutes when defender Jack Smith strode forward and drove narrowly wide.
Rob Hulse should have done better with Derby's first chance but his shot flew wide, before Chris Porter's header was comfortably dealt with by Lions goalkeeper David Forde.
But Millwall bossed the majority of the first half, with Dave Martin seeing a shot deflected wide and Smith fizzing another long-range effort past the post.
Chris Hackett then played in Lewis Grabban with a superb cross-field ball but the winger's angled drive crashed into the side-netting.
The hosts survived a scare on the stroke of half-time when Commons' shot took a wicked deflection off Jimmy Abdou and Paul Robinson had to head the ball off the line.
Rams manager Nigel Clough, a finalist with Nottingham Forest in 1991, threw on new loan signing Lee Johnson at the break in place of Lee Hendrie.
But the goal Millwall had been threatening eventually came in the 49th minute when Hackett swung in a high cross and Grabban tucked the ball past Rams goalkeeper Stephen Bywater.
However, the lead lasted barely three minutes before Commons capitalised on some hesitant Lions defending to equalise.
Tony Craig could only head Dean Moxey's throw-in to the feet of Commons and the Scotland midfielder lashed the ball home from 10 yards for his second goal of the campaign.
Both sides had chances to win the game with Martin heading Marc Laird's cross wide and Hulse finding the side-netting.
Millwall striker Steve Morison, who has scored seven goals in his last seven games, had a header hacked off the line and saw his follow-up shot blocked.
And in the dying seconds Morison put another shot inches wide as the Rams held out.