Yann Kermorgant fired home from the spot just after the half hour when Charlie Daniels drew a foul from Lee Martin, and Harry Arter doubled the Cherries’ lead in the 42nd minute after Callum Wilson beat the offside trap.
And things went from bad to worse at The Den for Millwall with 10 minutes remaining, as Shaun Williams was given a straight red card after an altercation with Bournemouth striker Wilson.
Bournemouth are now unbeaten in 14 matches in the league, while Millwall are sinking fast and the pressure is building on manager Ian Holloway following four defeats from their last five matches.
Millwall made seven changes following their 6-1 hammering at Norwich City on Boxing Day, with 18-year-old central defender Sid Nelson making his senior debut.
But it was the Cherries who made a fast start, as Kermorgant tested Millwall goalkeeper David Forde with a 25-yard free-kick.
Millwall almost took the lead on the quarter hour after good work from Scott Malone down the left, but his low cross just evaded Martin in the six-yard box.
Then the game turned in the 32nd minute, as the visitors were awarded a penalty when Martin brought down Daniels in the box, Kermorgant confidently tucking away the spot kick.
Forde was then shown a yellow card for dissent after the Championship leaders were awarded a corner, with the ‘keeper seemingly frustrated with Wilson’s theatrical fall in the box.
And the Cherries doubled their advantage three minutes before the break when midfielder Arter, celebrating his 25th birthday, netted his fifth goal in the last five matches.
Much to the frustration of the home crowd, Wilson raced down the right and was correctly ruled onside by the assistant referee. The Cherries striker then showed great awareness to square the ball to Arter, who swivelled before slotting home from close range.
Millwall came agonisingly close to pulling one back shortly before the hour, but Jimmy Abdou’s cross-shot narrowly evaded the head of Lee Gregory at the back post.
Bournemouth coasted through the second period, with the Lions rarely threatening Artur Boruc’s goal after their early flurry.
Substitute Dan Gosling should have added a third for the visitors though when he was found by Marc Pugh, but his shot crept wide of the right-hand post.
The home side continued to push forward but lacked the confidence and creativity to break through the Cherries’ determined defence.
There was final chance for the league leaders, when Simon Francis found the dangerous Wilson with a pin-point cross down the right, but the striker’s header was straight at Forde.
And Millwall’s misery was compounded when Williams was shown a straight red card for violent conduct 10 minutes from the end after appearing to stamp on Wilson.