David Prutton opened the scoring in the 37th minute and a purple patch of three goals in six second-half minutes turned the screw as Jermaine Beckford collected his seventh of the season in the 53rd minute and Jonathan Douglas went on to pounce twice.
Will Hoskins snatched Millwall's second away goal of the season on 65 minutes and Croatian Ahmet Brkovic, on loan from Luton, celebrated his debut with a 76th minute header.
There were reports of trouble outside the ground with missiles thrown after travel restrictions had been removed for Millwall supporters and rival fans had to be separated inside the stadium after Leeds' fourth goal.
The Lions began with a roar, Hoskins stabbing the ball into the Leeds net after only three minutes only to have his effort ruled out for offside.
But after Jamie O'Hara headed off target, Millwall were pinned back as Beckford, Seb Carole and Tresor Kandol all went close.
Leeds were rewarded in the 37th minute when French winger Carole turned Danny Senda inside out on the left and from Richard Shaw's headed clearance, Prutton's strike found the top corner for his second goal of the season.
Leeds' second goal was a gift. Goalkeeper Lenny Pidgeley's attempted clearance hit Kandol and found Beckford who chested it down and fired into the unguarded net.
Then Douglas scored in the 58th and 60th minutes. His first came from a superb turn and shot inside the box.
Then, after Shaw brought down Beckford on the edge of the penalty area, Douglas powered the free-kick through the defensive wall.
Millwall had some consolation when Hoskins beat Casper Ankergren from 16 yards and Brkovic headed his side's second, but Leeds fully deserved the points.