Easter, signed on loan from Crystal Palace 48 hours earlier, came on as a 57th-minute substitute and within 60 seconds had fired the Lions ahead.
And defender Lowry made it an unhappy Valley for Charlton again when he rifled in a stunning 35-yard free-kick six minutes later.
Millwall have taken their eye off the ball in the league since embarking on their cup run, losing seven of their last eight matches.
However, they returned to the day job in style to secure a valuable three points while condemning their south London rivals to a ninth home defeat of the season.
Millwall created the best of the few chances in the first half, with former Charlton loanee Dany N'Guessan firing an early effort wide.
Lions' keeper David Forde also won a race to the ball with Danny Haynes as the striker burst through, and then tipped a fierce Callum Harriott drive wide.
On the stroke of half-time Millwall's Chris Taylor held the ball up well at the far post and teed up Adam Smith but the defender, on for the injured Karleigh Osborne, poked wide from 10 yards.
Lawrie Wilson had a shot deflected over Forde's crossbar just after the break before Lions boss Kenny Jackett made his inspired substitution.
Easter, who scored on his Millwall debut during a previous loan spell in 2008, repeated the trick when he latched onto a clever through-ball from another new signing, Richard Chaplow, and flicked it under the advancing David Button.
It got worse for Charlton in the 64th minute when Bradley Pritchard fouled Smith some 35 yards out and Lowry curled a superb, dipping free-kick past a stunned Button for a memorable first goal of the season.
Charlton attempted to hit back and Yann Kermorgant headed over the top before substitute Ricardo Fuller was denied by Forde, but Wembley-bound Millwall held firm to ease their fears of being dragged into the relegation scrap.