Striker Chris Wood took his Lions tally to five since arriving on loan in September, with a brace either side of the break, while James Henry and Liam Trotter added goals after 78 and 87 minutes respectively to cement the win.
Huddersfield had Jack Hunt sent off on a straight red card late on, as Millwall celebrated only their fourth league win of the season.
Lee Novak took just two minutes to scare the hosts with a 20-yard strike that sailed over the bar.
Millwall opened the scoring when Darius Henderson won the ball in midfield and crossed from the left for a grateful Wood who made no mistake in sliding the ball in from 12 yards past Terriers keeper Alex Smithies.
Keith Southern attempted to restore parity for the visitors but could only fire over from close range, while Jermaine Beckford did well to make room in the box after 37 minutes but his tight-angle shot went wide.
Huddersfield controlled possession early in the second half with Millwall once again content to absorb pressure and wait for the opportune moment.
And it came after back-to-back Terriers corners, as a quick break by Millwall after 60 minutes saw them outnumber Huddersfield five to three as Henry fed Wood through for a neat finish and his second goal of the game.
Wood then turned provider, unselfishly laying off the ball for Henry from the left hand side, and the midfielder produced a calm strike deep inside the box to all but finish the game off.
Huddersfield's best chance of the second half came late on as Adam Hammill fired straight at Millwall keeper David Forde.
The Terriers' misery was compounded when Hunt was sent off five minutes from time for violent conduct, and Liam Trotter then rounded up Millwall's victory with a superb curling finish after 87 minutes.