Millwall ended in-form Blackburn's winning Championship run, snatching all three points with a late winner from substitute Mihailo Ivanovic.
A match which saw plenty of chances for both sides looked to be heading to a goalless stalemate.
But a 96th-minute header from Ivanovic secured the points for stand-in manager David Livermore on his debut in the dugout.
The goal was the first conceded by Rovers in six games and ended a run of six successive wins for John Eustace's promotion hopefuls.
They remain in fifth place, in the play-off positions, while Millwall climbed into 10th with their first win in seven matches.
Fans at The Den had to wait until deep into stoppage time for the deciding action in an encounter which should have seen the net ripple a few more times given the number of chances created.
Seconds away from the seventh of eight extra minutes, Ryan Leonard's measured cross from the right side of the Blackburn penalty area was met at the back post by Ivanovic, who nodded in from five yards out.
Victory will bring some welcome positivity to Millwall who have seen form slump with no wins in their previous six, and are dealing with manager Neil Harris stepping down after 10 months in charge.
On this showing and the fact the are proving stubborn opponents who have conceded no more than a goal in any of their past 19 league outings they may well be thinking of a push for a play off spot.
Defeat did not deliver doom and gloom to Rovers, who remain in the promotion-chasing mix and carved out enough chances to win this match.
Dom Hyam had the best of them in the first half, the centre-half heading narrowly wide after 23 minutes when he should have hit the target.
As both sides chased the game late on, Andreas Weimann nearly poked home a Ryan Hedges cross only for Lukas Jensen to smother his effort.
Yet on balance Rovers fans might concede that Millwall did enough to deserve the points.
Livermore's side could have reached the break ahead had Blackburn not kept their wits about them eight minutes before the break.
Femi Azeez had a right-foot shot blocked by Rovers keeper Aynsley Pears and when the rebound fell to Japhet Tanganga, his follow-up effort was cleared off the line by Joe Rankin-Costello.
After the break, Azeez was required at the other end, sprinting back 60 yards to smother a breakaway from Amario Cozier-Duberry as he shaped to shoot.
With eight of Millwall's previous 13 games settled by a solitary goal and with Blackburn having not conceded one in five matches, a goalless stalemate looked the most likely outcome.
Yet after Weimann was denied and, at the other end, Millwall's Macauley Langstaff headed over from close range, Ivanovic finally broke the deadlock.
'I'm proud of the efforts' - reaction
Millwall's stand-in manager David Livermore told BBC Radio London:
"Tough game against an opponent in form who are very good out of possession and made it very tough for us. I thought it was a scrappy game but territorially I think the ball was in their half more without us creating big chances.
"There is nothing better at The Den than scoring in the 90-plus whatever it was, so fantastic result for us.
"At 0-0 I just wanted to be positive. They had two very experienced centre halves in Dom Hyam and Danny Batth, I felt the physical presence might help us win the game.
"This week we have kept it really consistent. Before the game we talked about being a team who always wants to try to score the next goal whether we are winning, losing or drawing.
"At the minute I will be taking the game on Thursday [against Norwich] but I'm taking it day by day and if that changes, no problem at all."
Blackburn manager John Eustace told BBC Radio Lancashire:
"I thought it was a really good afternoon for both teams. Conditions were very tough and the way the boys battled was very pleasing. It is something we do week in, week out and obviously disappointing to concede in the last minute.
"Obviously it was disappointing to lose [Lewis] Travis with 10, 15 minutes to go but I still felt very comfortable. I didn't think they would score and I thought the way the boys defended was excellent.
"It looks like Lewis has done his hamstring so we will have to get a scan on that tomorrow and Harry Pickering took a blow to his knee, so we will have to have a look at that too.
"We played some good football today. We had two or three good chances in the second half which we didn't take. Overall I thought a draw was a fair result.
"I am proud of the efforts in the first half of the season and the run we have been on has been terrific. A lot of hard work has gone into it but there is going to be a lot of hard work going into the second half of the season."