DJ Campbell's brace helped Ipswich record their third successive win and end Millwall's 13-match unbeaten streak in the Championship.
Campbell, on loan from Queens Park Rangers, converted a flowing move from close range to put the home side ahead with his seventh goal in 12 games.
He then turned provider by chasing down Danny Shittu to gain possession and feed Daryl Murphy who swept home.
Campbell scored his second from the penalty spot after Shane Lowry handled.
It means Ipswich have recorded three victories in a row for the first time in 10 months and Mick McCarthy, who replaced Paul Jewell in November, has won five of his eight games at the helm.
Despite the convincing win they still remain just five points clear of the relegation zone, while the defeat leaves Millwall in seventh place.
Millwall began the Portman Road encounter as the second tier's form side having won four of their last five matches, boasting four successive clean sheets in the process.
And they should have taken the lead after enjoying the better of the opening half-hour.
New Zealand striker Chris Wood had an early goal ruled out for offside before heading a great chance over the Ipswich crossbar, while Town goalkeeper Stephen Henderson denied efforts from Andy Keogh, Liam Trotter and James Henry.
Ipswich took control when Campbell turned in Aaron Cresswell's superb left-wing cross, before Murphy doubled their advantage just five minutes after the restart.
Millwall goalkeeper David Forde tipped Lee Martin's vicious free-kick over the crossbar, but he could not prevent the Tractor Boys netting a third through Campbell's penalty.
Lions defender Shittu cleared off the line to deny Martin scoring a late fourth.
Ipswich manager Mick McCarthy:
"Everyone is starting to talk differently, rather than about staying up, but I'm just happy with the points. I was delighted and thought we deserved it, although I wouldn't say it was easy.
"The lads are knackered and really put a shift in. Teams only play as much as we allow them to and I thought our boys were terrific in stopping Millwall play.
"Other people will be interested in DJ Campbell because he's scoring goals. He wants to sign for a club permanently and we'd like to take him, but there will be mitigating circumstances.
"I left (QPR boss) Harry Redknapp a message the other day but I think he's got plenty on at the moment."
Millwall boss Kenny Jackett:
"Wood's (disallowed) goal from Lowry's pass was clearly onside, having seen it again, but we've gone in 1-0 down.
"Their keeper has made three very good saves and we've had an onside goal disallowed.
"We could have been one or two up in that period, but instead we found ourselves 1-0 down.
"The start of the second half was poor for us. Two poor goals took the game away from us."