City needed a win to keep their play-off hopes alive, while the Lions just needed a win.
Millwall winger Paul Ifill stumbled when clear through and the Canaries capitalised on that good fortune when skipper Malky Mackay thumped home an unstoppable half-volley after Steven Reid had failed to get good distance on a headed clearance.
And with the Lions still reeling from that eighth minute setback, City almost added a second as Zema Abbey benefited from sloppy defending but found his route to goal blocked by keeper Tony Warner's legs.
Neil Harris' shot on the run hinted at a comeback for the home side and Andy Roberts almost found the target as he latched on to Robert Green's weak punch.
But Millwall's brief period of resistance ended courtesy of a defensive error that effectively handed City the points.
On-loan Leigh Bromby launched a long range throw into the area and Darren Ward's attempted control merely presented Abbey with a simple finish.
It wasn't that Norwich were that much better than their opponents, they just didn't have to be.
Some solid second-half defending nullified anything Mark McGhee's men had to offer.
And in truth the Canaries should have added to their lead after Green had instinctively kept out Steve Claridge's close-range volley.
Abbey glanced a header wide and Steen Nedergaard struck the underside of the crossbar after Warner had kept out Mark Rivers' drive.
The lively Abbey contrived to miss an open goal before being replaced late on by Iwan Roberts, but he can be pleased with his evening's work as can the rest of his team-mates.