The Lions had to play the entire second half a man down after young defender Sid Nelson was sent off for a ridiculous late challenge in first-half stoppage-time.
But Ian Holloway’s new-look side could still have snatched all three points had Lee Gregory and Jos Hooiveld converted chances after the interval.
However, Rotherham’s win over Bolton leaves Millwall marooned in the drop zone, now five points from safety.
Holloway has overhauled his under-performing squad in a bid to drag them away from danger and named five of his six January signings in the starting line-up.
One of them, Diego Fabbrini, had the first shot on target after winning a free-kick 20 yards out and then firing it too close to Royals goalkeeper Adam Federici.
Watford loanee Fabbrini was causing Reading plenty of problems down the left and one jinking run into the area took him past four defenders before his cross was plucked out of the air by Federici with Gregory looking to apply the finish.
Millwall’s clearest chance of the first half did fall to Gregory but the former Halifax striker’s shot from the edge of the area fizzed narrowly wide.
Reading had their opportunities too, however, with Pavel Pogrebnyak glancing one cross from Jordan Obita wide and another straight at Lions keeper David Forde.
Pogrebnyak felt he could have had a penalty when he was challenged in the area by Nelson but referee Andrew Madley was not interested.
Millwall’s evening took a turn for the worse seconds before half-time when 19-year-old Nelson, making only his fifth appearance for the Lions, launched himself into a crazy late tackle on Michael Hector near the halfway line.
The offence was compounded by the fact the referee’s whistle had already gone for an earlier foul, and Madley duly pulled out a red card as a distraught Nelson ran straight down the tunnel.
After the break, Reading, sensing the chance to ease their own fears of being sucked into relegation trouble, piled forward and Forde had to beat away shots from Obita and Daniel Williams.
But Millwall had chances to secure an unlikely win with Gregory heading Dan Harding’s cross wide and on-loan Southampton defender Hooiveld, on his debut, nodding Shaun Williams’ free-kick off target.
They were indebted to Forde for a point, however, when the keeper scrambled a shot from Royals sub Simon Cox round a post late on.