Millwall forward Aiden O'Brien has scored five goals in 27 appearances for the Lions this season
Preston left it late to deny Millwall a fourth win in five home league games.
Aiden O'Brien broke the deadlock two minutes before the break, steering Jed Wallace's low cross from the right flank into the bottom corner.
Callum Robinson equalised with 10 minutes left, chesting in from close range after latching on to Greg Cunningham's ball in from the left.
And The Lilywhites could have stolen all three points, but James Meredith blocked Daryl Horgan's shot.
Millwall's bid for survival, following promotion from League One last season via the play-offs, has been built on their home form so far this season.
The Lions have picked up 25 of their 31 points in south-east London, but are yet to win away from The Den since their return to the Championship.
George Saville drilled wide as the hosts looked to extend their lead after the break, while Lee Gregory thought he had scored a second, only to be flagged offside.
Robinson, meanwhile, was a constant threat for the visitors, testing Jordan Archer from range and sending two other efforts off target in the first half before finally sneaking in behind the defence to grab the leveller.
Preston are 10th in the table, just two points off the play-off places, having only lost one of their last 11 league matches, while Millwall remain 16th, seven points above the relegation zone.
Milwall boss Neil Harris said:
"When you're 1-0 up at home at half-time, it's disappointing not to get three points.
"But they're a good side, a competitive side, an aggressive side in the way they press and the way they play. They're a team that'll be in the top six come end of the season.
"But I thought there were two very fine calls today. Both should have gone our favour. Ben Pearson should have been sent-off because it was identical to Jake Cooper's second yellow card at Sheffield Wednesday earlier in the season, so I don't understand that one.
"Secondly, Lee Gregory goes through one-on-one and was clearly three yards onside. I've watched it through two different angles and he was clearly onside. Two fine margin decisions at 1-0 that could have affected the outcome of the game."
Preston manager Alex Neil said:
"I think it's harsh to say Ben Pearson should have seen a red card. I thought the first booking was really disappointing and he didn't really deserve it, and I think Ben's frustration was the same as mine.
"Within a game, there are a lot of 50-50 decisions and you're hoping they're going to get spotted evenly. I feel we didn't get our fair share.
"There was one incident where the ball was on the sideline and the linesman said it was out. I was right next to it and it was nowhere near out. You get them from time to time but he certainly did not deserve a red card. I don't think there were any dirty tackles in the game whatsoever."
