Millwall made it an impressive four wins from four to revive their play-off hopes and knock Sheffield United out of the top six.
Jake Cooper’s towering header at the back post in the 63rd minute marked a frustrating day for the away side in a game of little quality.
The win puts Millwall within five points of the play offs, in which the Blades have been replaced by Luton, following their first defeat in nine games.
The Lions suffered an early setback, with Mason Bennett forced off with a hip injury within the first quarter of an hour in a half of little action.
Jack Robinson’s cross-cum-shot just after the break was the first time either keeper was called into any real action, with Bartosz Bialkowski needing to be alert to tip over.
At the other end, Wes Foderingham was less than convincing in dealing with Danny McNamara’s deflected, long range drive, which the Blades’ defence managed to scramble away.
A dull afternoon at The Den finally burst into life, after Cooper peeled away unmarked at the back post and was able to powerfully head home Jed Wallace’s dinked free-kick.
The visitors attempted to hit back and largely dominated the rest of proceedings but struggled to create anything clear-cut.
Some Blades’ fans were left bemused at the decision to leave midfielders Sander Berge and Morgan Gibbs-White out of the starting lineup, after their recent positive form.
Both Berge and Gibbs-White did come on in the second half but failed to make an impact.
A rare positive on a disappointing day for the Blades was the impressive league debut of 19-year-old centre-back Kyron Gordon.
“I support Sheffield United, I support Sheffield, so when I knew I was starting I was buzzing,” Gordon said.
“We didn’t get the result we’d have liked, but for me personally, it’s a massive achievement.
“Hopefully I can put that shirt on every week”.
Millwall manager Gary Rowett was delighted to complete the double over the Blades and was full of praise for away team after the game.
He said: “Sheffield United are a good side.”
“I have watched a lot of them recently and I think they are probably as good as any other team behind Fulham in this division.
“I expect them to be challenging come the end of the season, probably even for the top two positions.”
Sheffield United had 62% possession and more shots on goal, which left manager Paul Heckingbottom lamenting the fine margins of the contest.
“We probably shaded it in terms of chances, efforts and possession, but we were not at our best,” he told SUTV.
“It was a nothing game, so that’s the frustration, it looked like two teams who had played Wednesday night”.
Heckingbottom will be hoping his side can bounce back on Friday night, when they host fellow play-off hopefuls Nottingham Forest.