Ashley Ward and Andy Gray provided the tonic for United's hangover as Neil Warnock's ailing side stopped the rot against Dennis Wise's in-form outfit.
The two strikers, once team-mates at Bradford City, struck in a lightning four-minute spell as United bounced back from a miserable run.
With both teams still involved in the FA Cup, the match could have been a dress rehearsal for the play-offs following Millwall's transformation since Wise's move into management.
Millwall, with a formidable run of one defeat from 14 games, were content to flood the midfield with the loan figure of Danny Dichio up front in a niggly game with little quality.
United, with just one win from their last six league games, found a way through on ten minutes, Ward flicking a Rob Kozluk long throw into the goalmouth where on-loan midfielder Carl Robinson headed over from eight yards.
Welsh international defender Robert Page went close 17 minutes later heading inches wide of the far post before Millwall, pegged back in their own half, squandered a chance of their own.
Page and Mike Whitlow failed to head clear but, from ten yards Tim Cahill scuffed his shot and the ball rolled safely to keeper Paddy Kenny.
Ward almost broke the deadlock five minutes after the restart steaming in to meet a Michael Tonge cross as Andy Marshall pushed his downward header round the post.
Tonge went closer minutes later with a free-kick from the edge of the area, the ball being smothered by the diving Marshall.
The breakthrough came in the 58th minute, the hardworking Phil Jagielka setting up the goal with a low cross from the right into a packed goalmouth where Ward bundled the ball over the line from close range.
Gray, signed last week from Bradford, grabbed a second four minutes later after a skirmish involving every player apart from Jagielka resulted in Kenny, Ward and Darren Ward being booked.
Marshall spilled Tonge's free-kick and Gray lashed in the loose ball from eight yards.
Midfielder Paul Ifill gave Millwall hope with a spectacular 25-yard half-volley with 20 minutes to go but Kenny foiled substitute Neil Harris as the London side failed in their desperate search for an equaliser.