Millwall ended their three-game home draw streak with a win that lifted them to fourth in the Championship.
Tom Bradshaw scored twice and forced an own goal from Cameron Humphreys as the Millers fell into the relegation zone for the first time this season.
The defeat increases the pressure on Rotherham manager Matt Taylor, who has lost nine out of 15 games since taking charge in October.
Rotherham were without half a dozen key players and Millwall took the opportunity to return to play-off positions with just their third win in nine games.
Bradshaw gave the Lions, without a home win since October 22, the perfect start as he rose above taller defenders to lead the fourth-minute opener from a Callum Styles free-kick.
Goalkeeper Viktor Johansson kept Rotherham in the game with fine saves from Styles, George Honeyman, Shaun Hutchinson and Bradshaw, while the latter blazed over the bar when set up again by Styles.
Conor Washington fended off a great chance to level just before half-time when he was sent home by Dan Barlaser’s fine pass, rounding goalkeeper Bartosz Bialkowski but firing with only one defender on the goal line to beat.
Johansson was eventually beaten by Humphreys’ big deflection on a volley shot from Bradshaw.
The forward made six for the season as Zian Flemming and Andreas Voglsammer led a counterattack, and when the latter’s shot was parried by Johansson, Bradshaw grabbed the rebound.
Millwall are now unbeaten in eight games at the New Den, but the misery is getting worse for Taylor.
Millwall manager Gary Rowett said:
“We changed the game really well, we had some brilliant runs on the ball and it felt like a really proactive performance.
“I can’t think how many times George Honeyman has fallen behind with a great run or Vogey [Andreas Voglsammer] and Bradder [Tom Bradshaw].
“We worked incredibly hard to open up the space and it made a big difference for us today.
“It’s been difficult for Rotherham. I’ve been following their last few games and they’ve suffered some key injuries which have made it very difficult for them to stay as competitive as they would like.
“They worked incredibly hard – there were a few moments where there were few warning signs. We played so well in the first half to go 1-0 up, but we should have had more than one .”
Rotherham manager Matt Taylor told BBC Radio Sheffield:
“There is no shortage of effort in this group of players, but there is a lack of quality and a lack of belief in what we can compete with, and we came up against a good team.
“There has been a softness, a weakness, a naivety in our defensive play since our return from the international break.
“No disrespect to the outside of the team, that’s the nature of the beast at this level. I can’t have the fact that they don’t try – they give their all but don’t execute what which is necessary to keep us in the game.
“Why have we been relegated three times in six years to get to this level? We need to find players who can do what is necessary to keep us in this league – right now it feels like a form of relegation.
“The wheels are in motion, we’re working towards the goals and trying to get them in. We’ll have to be quite creative with that because more than realistically it’s a loan market and we’re limited by the number of loan players we have. can have at the same time.
“We also have to be creative with free agents, but we knew that before I took the job. We have to get some safe stuff, experienced players with other clubs at this level.”
