Neil Harris's fine brace led Millwall to a sixth win in seven games to keep the Lions' slim play-off hopes alive.
The hosts had to survive an early storm with keeper Lenny Pidgeley twice saving from close range, first from Chris Lumsden and then Danny Livesey.
And they made Carlisle pay for their profligacy when Harris controlled Tony Craig's cross and slotted in neatly.
Harris wrapped it up minutes after his shot was cleared off the line, poking in after Craig's shot had hit a post.
Millwall: Pidgeley, Senda, Robinson, Shaw, Craig, Ardley (Dunne 46), Elliott, Brammer (Hackett 86), Brighton, Harris, Byfield.
Subs Not Used: Day, Zebroski, Hubertz.
Goals: Harris 52, 82.
Carlisle: Westwood, Raven, Livesey, Murphy, Aranalde, Lumsdon (Hindmarch 84), Thirlwell, Joyce (Vipond 90), Jeff Smith, Garner, Johann Smith.
Subs Not Used: Williams, Gray, Arnison.
Ref: P Joslin (Nottinghamshire).