Carson was caught in no-man's land when he was beaten by Defoe, who, in front of the England manager Fabio Capello, scored his sixth goal since his January move from Tottenham.
Villa responded positively, with Ashley Young's trickery and pace causing Pompey problems. But for all their attacking promise the visitors were toothless in attack.
John Carew, having done the hard work by beating two defenders, was guilty of missing a great opportunity 10 minutes before the break.
And Carew was to rue his profligacy when Portsmouth doubled their lead.
Carson came rushing out of his area to clear the danger posed by Muntari's through ball. The keeper kicked the ball straight at Reo-Coker and, to everyone's surprise, the ball bounced off the midfielder's toes into the back of the net.
Villa should have halved the deficit immediately after the break, but Gabriel Agbonlahor failed to connect with Shaun Maloney's cross for what would have been a simple tap-in.
Marlon Harewood came on for Maloney as Villa boss Martin O'Neill sought to out-muscle the Portsmouth defence.
But the home side stood firm despite a bombardment of free-kicks into their box.
Portsmouth should have scored a third only for Niko Kranjcar to shoot wastefully wide from a promising position.
Villa finally found the back of the net - but it was disallowed as Harewood was offside.
Minutes later and Young's free-kick forced a fine reflex save from ex-Villa keeper David James, who was making his 161st consecutive appearance in the Premier League.
Portsmouth were then reduced to 10 men when Muntari received a second yellow for a foul on Villa substitute Moustapha Salifou.
Yet, even with a man advantage, Villa were unable to break Pompey's defence and, three minutes before the end, Villa's frustrations came to a boiling point when Mellberg was sent off for a second yellow card.
• Aston Villa manager Martin O'Neill:
"Luck deserted us but it's all about winning games and we've got to get back to that.
"We won't give up. There are still eight games to go and if we are capable of winning most of them.
"There are plenty of twists and turns left even though the margin for error is growing less."
• Portsmouth manager Harry Redknapp:
"We've gone above Villa into sixth place now but Everton are the team to catch for fifth and we'll keep trying.
"We did well because Villa set us a lot of problems, particularly from set-pieces.
"They're such a big side that it was bound to be that way but without Sol Campbell we defended really well."
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