Neil Harris and Ahmet Brkovic struck as Millwall eased away from the drop zone with a second successive home victory.
Harris lobbed just over early on before Ben Williams denied Gary Alexander, while Crewe's Chris McCready hit the post from Mark Carrington's corner.
The Lions went ahead when Harris smashed home a penalty after McCready pulled the striker's shirt.
Substitute Brkovic latched onto Dave Brammer's quick free-kick before the Lions closed out a comfortable win.
Millwall: Pidgeley, Senda, Robinson, Whitbread, Bignot, Hodge, Brammer, Simpson, Fuseini, Alexander (Brkovic 38), Harris (Akinfenwa 86).
Subs Not Used: Edwards, Barron, Shaw.
Goals: Harris 45 pen, Brkovic 53.
Crewe: Williams, Woodards, Roberts, Baudet, O'Connor, McCready, Moore (Lowe 77), Carrington (Bennett 77), Abbey, Barnard, Church (Maynard 71).
Subs Not Used: O'Donnell, Rix.
Ref: Trevor Kettle (Rutland).