One defeat in 11 games, and that at Anfield, had seen Neil Warnock's Blades close a seemingly insurmountable gap on runaway leaders Portsmouth and Leicester City.
In Millwall they faced opposition beginning to find form of their own, an FA Cup replay against Southampton looming large after a great display at St Mary's and unbeaten at the Den in nine games.
United' Premiership pretensions has seen them set their sights on bigger fish than Mark McGhee's men, while the Lions, despite their mid-table position, lay only three points from a play-off place themselves.
The stakes were high, not that the first 45 minutes gave any indication of that fact.
Neither goal came under any serious threat, Wayne Quinn's skidding free-kick providing a rare moment of excitement while Michael Brown's shot at least forced Lions keeper Tony Warner into a save.
Millwall full-back Robbie Ryan had his side's only goal attempt in an instantly forgettable first half. But thankfully the pace picked up after the break, coinciding with the introduction of Lions substitute Christophe Kinet.
The winger injected some much-needed urgency and his looping shot almost broke the deadlock. It was the Belgian's deep cross that was headed back across goal by Steve Claridge that gave Paul Robinson a chance from three yards but his header bounced wide.
That miss was forgotten however 30 seconds later as Paul Ifill cut in from the left wing and, despite the best efforts of a bumpy pitch, he unleashed a shot into Paddy Kenny's bottom corner.
The winger had scored a similar stunning strike when the two teams met at Bramall Lane earlier in the season but the Blades had failed to learn their lesson.
The goal gave the Lions a boost enabling them to keep United comfortably at bay.
Wayne Allison and Steven Kabba replaced Dean Windass and Tommy Mooney with 25 minutes to go, but they had just as little effect against a well marshalled Millwall rearguard.
Dennis Wise's 40-yard lob could have beaten Kenny and the midfielder was clear when referee Brain Curson blew for full time.
But with the points secured Wise was the first to smile, closely followed by his manager Mark McGhee who now sees his side a genuine promotion contenders.