Irish striker Andy Keogh grabbed a brace to help Millwall clinch their fifth successive Championship victory.
Keogh turned in Liam Trotter's cross at the far post inside three minutes before ex-Ipswich midfielder Trotter fired in from the edge of the area.
Euro 2012 hopeful Keogh slotted home near the end to cap an excellent April for the Lions.
Ipswich's Grant Leadbitter failed to reduce the deficit to 2-1 when Maik Taylor saved his second-half penalty.
Victory moved 16th-placed Millwall within two points of Ipswich, with neither side involved in the Championship's promotion or relegation picture.
Millwall were again without injured top scorer Darius Henderson, but Keogh more than filled the void.
Home midfielder Andy Drury should have equalised moments after the Lions' opener but blasted a golden chance wide.
Michael Chopra also saw his fizzing effort from the left come back off a post in the second half, with substitute Jason Scotland unable to turn home the rebound.