The Lions boosted their survival hopes and dealt a blow to Derby's automatic promotion chances when they pulled off a shock win on Saturday.
A second-half header from Morison was enough to give Millwall only their second away win of the season and condemn Derby to their second defeat in eight days.
It had started so well for the home side with Craig Bryson, who scored a hat-trick against Millwall in September, delivering a free-kick that Patrick Bamford headed narrowly wide in the fifth minute.
Even without their suspended top-scorer Chris Martin, Derby looked a threat every time they went forward and Millwall goalkeeper David Forde had to spread himself to block a Conor Sammon shot from close range in the eighth minute.
It was all Derby but Millwall showed they should not be underestimated in the 19th minute when Morison chested the ball down and Ed Upson created space to hook a shot wide from 15 yards.
The visitors were starting to get into the game and Morison set up another chance in the 27th minute when he pulled the ball back from the left, but Scott Malone fired well over.
Bamford curled a shot wide after cutting in from the right and Derby had a good chance at the start of the second half when Bamford won the ball just inside the Millwall half and played in Simon Dawkins, whose low shot was saved by Forde.
Dawkins cut Millwall open in the 53rd minute when he jinked past two defenders but Forde was out quickly to beat away his drive and the visitors lost skipper Danny Shittu two minutes later.
But the visitors stunned Derby in the 61st minute when Lee Martin crossed from the left and Morison stole in to head into an empty net from two yards.
It brought an immediate response from Derby head coach Steve McClaren, who brought on two attackers in Jamie Ward and Johnny Russell but it was Millwall who had another chance to score again.
Morison was again involved, playing in substitute Martyn Woolford on the Derby left but his low shot was struck straight at Lee Grant who saved easily.
The Derby fans were becoming increasingly frustrated at what they saw as time-wasting but Millwall had a wonderful chance to win the game in the 82nd minute when Jake Buxton lost the ball and Morison raced clear, only for Grant to deny him with a brilliant one-handed save to his right.
A roar went round the stadium when the board went up showing seven minutes of added time and Richard Keogh headed wide from a good position as Millwall defended desperately.
Derby had a chance to salvage a point in the final minute of added time when Ward floated in a free kick from the left but Keogh's diving header flashed wide and seconds later Millwall were celebrating in front of their fans.