The striker's fifth of the season secured the derby-day spoils for Palace, a year after they hit rock bottom at the same ground.
Last New Year's Day, Palace sacked their manager George Burley after a 3-0 defeat by the Lions left them deep in relegation trouble in the Championship.
Millwall were, at the time, flying high and dreaming of an unlikely bid for promotion to the top flight.
But fast-forward a year and they now find themselves hovering above the drop zone while Palace, reinvigorated under Dougie Freedman, are back in the play-off mix.
The Eagles also have a Carling Cup semi-final against Cardiff to look forward to later this month, but they proved they have not taken their eye off the ball in the npower Championship with a hard-earned win - their first at The Den since 1996.
The goal came after 23 minutes of typically scrappy derby action, from a slick move started by Wilfried Zaha.
Palace's teenage starlet momentarily escaped the clutches of his tactile marker Tony Craig and played a clever ball inside to Chris Martin.
Norwich loanee Martin spotted Darren Ambrose's dart into the area, played a perfect through-ball to the midfielder's feet and his first-time cross was prodded in by Easter.
The visitors could have made it two on the half-hour when Ambrose's corner fell to Mile Jedinak at the far post but his shot was tipped over by Steve Mildenhall.
And moments later Millwall missed a gilt-edged chance to equalise when Dany N'Guessan met Hamer Bouazza's cross two yards from goal but somehow planted his header into the ground, high and wide.
Palace keeper Lewis Price then foiled Liam Trotter but Mildenhall was the busier keeper at the end of the first half, keeping out Ambrose's free-kick and Martin's follow-up and then denying Zaha low down.
After the break Trotter was inches too high with a header from Bouazza's corner and the midfielder also blazed over from the edge of the area.
The Lions thought they had equalised with 15 minutes to go through their skipper Paul Robinson.
But Palace are a far more resolute bunch than the one that showed up a year ago, and Robinson's header was nodded clear from under the crossbar by Jonathan Parr to ensure the Eagles made the short trip home with all three points.