The Wales striker may have struggled against England last weekend, but he shot down the Tigers with a superb first-half finish on Saturday.
Morison's 16th goal of the season saw Millwall leapfrog their hosts in the table to remain in touch with the top six and still dreaming of back-to-back promotions.
City are just a point behind but games are running out and they need to turn around the home form which sees them without a win at the KC Stadium since February 12.
Hull made a brisk start and Matty Fryatt's neat turn and cross had to be cleared by Jack Smith with Robert Koren waiting to pounce.
Andy Dawson, making his 250th league appearance for the Tigers, then swung in a free-kick which Fryatt headed wide.
At the other end, on-loan Tottenham winger Andros Townsend created Millwall's first chance when he crossed for James Henry, who drilled his shot into the side-netting.
But after an otherwise dull first half hour, Morison lit up the game in the 33rd minute.
Liam Trotter won possession from James Harper in midfield and played a clever pass through to Morison, who expertly dinked the ball over City goalkeeper Brad Guzan and into the net.
Guzan did better when he held another shot from Henry while, as half-time approached, groans filled the air as Aaron McLean fired wastefully wide.
Hull boss Nigel Pearson introduced former Lions loanee Jay Simpson at the break in a bid to pep up his side's anaemic attack.
But Millwall remained the more threatening side and Morison could have doubled his tally when the ball fell to him inside the box, only to sky his shot.
Lions goalkeeper David Forde had to make a save towards the hour mark when he kept out a shot from Koren.
Moments later the lively Townsend came agonisingly close to making the game safe for the visitors. Corry Evans fouled Tamika Mkandawire just outside the area and Townsend curled the free-kick inches wide.
Hull belatedly went on the attack and Koren scooped another effort over the bar before Simpson headed a cross from Fryatt wide.
Former Lions youngster David Amoo, on as a substitute, had a chance to snatch a point but put his shot too close to Forde before Simpson slid an injury-time opportunity wide.