Lions defender Alan Dunne joined the attack in a desperate last-ditch raid and duly kept the visitors' League One play-off hopes alive with a blistering left-foot shot from the edge of the box.
Walsall had only themselves to blame, after having conceded a crazy first goal, they then had full-back Matt Richards sent off in the 64th minute for a second bookable offence.
The Saddlers looked to be heading for only their sixth home win of the season on Saturday as they built a two-goal lead. Alex Nicholls burst through to drive a low cross-shot into the net before Richard Taundry smashed a glorious rising drive past David Forde to double their lead after 27 minutes.
But Millwall were handed a lifeline soon after when goalkeeper Rene Gilmartin got in a mix up with skipper Jamie Vincent and smashed the ball against the legs of visiting striker Steve Morison, who coolly picked up the loose ball and rolled it into an empty net for his 12th of the season.
Millwall, though, had much the better of the second half, particularly when Walsall were down to 10 men, and Morison saw a goalbound header cleared off the line by Clayton McDonald before Dunne struck.