A quick-fire double from Tom Bradshaw saw Kevin Nugent's Under 23 side stretch their unbeaten run to seven matches with a victory over 10-man Watford at The Den.
Bradshaw struck twice - the first from the penalty spot - within just a minute of one another early in the second half as The Lions maintained their solid form in the Professional Development League South.
Nugent was able to field a strong side for the encounter, with Shaun Hutchinson playing his first competitive football since August - getting 45 minutes under his belt - and Bradshaw leading the line. Jordan Archer again started between the sticks looking for another clean sheet, whilst Jem Karacan anchored the midfield.
The Hornets also started several first-teamers, with Miguel Britos and Marvin Zeegelaar featuring.
An even first half saw limited chances, with the defences coming out on top. Watford passed up the first clear-cut attempt 17 minutes in when Ryan Cassidy diverted a low Dion Pereira cross onto the post, before Archer recovered well to clear off the line.
Joe Wicks and Bradshaw then had chances for The Lions denied by visiting goalkeeper Sam Howes as the first 45 minutes ended goalless.
The second half swung quickly into Millwall's favour, as Rob Harvey excellently got in behind and was brought down for a penalty kick after 49 minutes.
Wales international Bradshaw stepped up and lashed home from 12 yards, sending Howes the wrong way.
Before Watford were able to recover, The Lions found dangerous space again, and found the net once more. Following a showing of great strength from Isaac Olaofe, the striker played it to Bradshaw who struck a low drive beyond Howes for 2-0.
The Hornets' comeback hopes were dashed 20 minutes from time, as Zeegelaar was dismissed for a hand to the face of Millwall captain James Brown following an altercation.
Despite Watford's best attempts including a close effort from Dom Hutchinson outside the area, The Lions comfortably held on to see out the points.
Next up for Nugent's youngsters is an away clash a week on Monday against Queens Park Rangers.
Millwall: Archer, Brown, McNamara, Karacan (Parker 71'), Hutchinson (Debrah 46'), Stojsavljevic, Harvey, Wicks, Bradshaw (Alexander 71'), Olaofe, Saunders-Henry. Subs not used: Sandford, Mitchell.