A first-half goal from Tommy Smith was enough to win a scrappy match which also ended badly for Millwall's Jody Morris who was stretchered off with a serious knee injury.
Morris fell awkwardly in the 28th minute and it was later confirmed that he will not play again this season.
Derby's on loan full-back Alan Wright is also unlikely to figure in the last four games after he damaged a hamstring in the second half.
But overall Derby's future looks considerably brighter than the Londoners.
Millwall now need to win their remaining four games and hope that other clubs fall apart, but the reality is they will almost certainly be playing in League One come August.
They did not create a clear cut opening and the forward line looked toothless following the recall of on loan midfielder Colin Cameron to Wolves.
With Morris also ruled out, they struggled to really threaten a Derby team that lacked any real rhythm but still managed to open up the Millwall defence.
If Kevin Lisbie had been fully fit instead of nursing a hamstring strain he would probably have accepted at least one of the several chances that fell to the on loan Charlton striker.
But Derby were able to hang on to the lead that Smith gave them in the 31st minute when he exchanged passes with Lisbie and turned inside to curl a right-foot shot from the edge of the area inside the left-hand post.
It was a smart finish and Derby should have had more goals but Lisbie was foiled by the legs of Colin Doyle two minutes later.
Millwall flickered briefly towards the end of the first half when Derby survived a goalmouth scramble, but Smith wasted a great chance to score a second after Mounir El Hamdaoui had a shot parried by Doyle.
The second half was a story of Derby creating openings but failing to take them with Lisbie denied by a last ditch tackle by Matt Lawrence before he shaved a post with a low shot.
Substitute Adam Bolder was also through only for his control to let him down but Millwall did not really pose a threat and Derby were able to secure a fourth straight win at Pride Park.