Andy Keogh marked his return to the Millwall starting XI with the winner 14 minutes from time against Derby.
Liam Feeney deservedly put the hosts ahead with a first-time shot and though an offside flag was raised, referee Graham Salisbury awarded the goal.
Millwall wasted chances to extend their lead and the Rams levelled as 17-year-old Will Hughes fired in from 20 yards.
But Scott Malone laid Feeney's long ball off to Keogh, who neatly tucked in his fifth goal of the season.
Republic of Ireland striker Keogh only started because of an injury to top-scorer Darius Henderson, and Millwall were also without winger James Henry, plus manager Kenny Jackett, who was struck down by illness.
They dominated the early exchanges, however, and Chris Wood had already blazed over when Feeney, who replaced Henry, scored from Chris Taylor's cross.
There was controversy surrounding the goal as celebrations were cut short by the flag, but referee Salisbury consulted with his official and then signalled for the goal.
As Millwall continued to press, Keogh curled the ball into the area but Wood was inches away, and the start of the second half saw Feeney shoot narrowly wide and Lowry head over from a corner.
The Rams had created little but Craig Bryson laid the ball off for Hughes to crash in his second senior goal.
However, Keogh's strike ensured Millwall stretched their unbeaten run to nine matches, with Lions goalkeeper David Forde saving a late drive from Michael Jacobs.
Millwall assistant manager Joe Gallen, standing in for Kenny Jackett, who missed the game with flu:
"I had an idea on Thursday he wasn't going to make it but carried on with what he was going to do.
"Kenny picked the team and discussed what training we are going to do and told me what the team needed to do.
"The team has been defending and running and working and blocking for the previous nine games. It's been a big feature of our performances."
Derby manager Nigel Clough:
"We thought we should have had a penalty at the end and one at the end of the first half.
"We can't control that. We can only control our own performance and we thought we deserved to go in level at half-time."