On a bitterly cold and windy morning in Arizona's high desert, American Mahan sealed victory with a matching birdie at the driveable par-four 15th, sinking a four-footer after Englishman Poulter had rolled in a seven-foot putt.
Mahan, who delivered a superb display of chipping in difficult scoring conditions, will face compatriot Matt Kuchar in the title match later on Sunday at Dove Mountain's Ritz-Carlton Golf Club.
Kuchar came from one down after three holes to beat Australian Jason Day 4&3 and will appear in the Match Play final for the first time after reaching at least the last eight for three years in a row.
"I'm excited to have a chance back at Hunter," said Kuchar, who was beaten 6&5 by Mahan in last year's quarter-finals at Dove Mountain. "He's a fantastic player, a fantastic ball striker.
"Being able to control the spin and trajectory of your ball in this wind is an important thing, so it'll be another tough match. He's proved himself a great player in this format."
Mahan, who won last year's title with a 2&1 victory over Rory McIlroy in the final, is aiming to emulate Tiger Woods (champion in 2003, 2004 and 2008) as the only back-to-back winner of the elite event.
"I know I'm playing well," said Mahan, who has completed 166 consecutive holes at the Match Play without trailing in a run dating back to the opening round last year.
"That's the best thing I have going for me. My short game and putting are great right now, and those are things I'm probably going to have to lean on this afternoon."
Mahan, who had been all square with matchplay specialist Poulter after four holes, went 1up after the Briton took four shots to reach the green at the par-four fifth.
The American doubled his advantage at the par-five eighth, where he chipped from below the green to two feet, and remained 2up after hitting a superb pitch from 76 yards to two feet for a conceded par at the ninth.
After ending up behind the green off the tee at the par-three 12th, Mahan spectacularly chipped in for birdie to go 3up on Poulter as he maintained a tight grip on the match.
The Englishman also lost the par-four 14th, recording a bogey there after finding the right rough off the tee and missing the green to the right with his approach to trail by four with just four holes remaining.
One hole later, Mahan ended the match after his drive ended up just off the green and he chipped up to four feet to set up his fifth birdie of the day.
Mahan is aiming to emulate Woods, with 16, and Australian Geoff Ogilvy, with three, as the only players with more than two World Golf Championships (WGC) titles. Mahan also won the 2010 Bridgestone Invitational at Firestone.
Kuchar took early control against an out-of-sorts Day with birdies at the sixth, where he sank a 12-footer, and at the eighth, where he chipped to within two feet of the flagstick, to go 2up.
The American also won the par-four ninth after Day bogeyed but then slipped back to 2up by making a mess of the 10th where his drive sailed left into the desert scrub.
Day also lost the par-five 13th, where he went from the left rough off the tee to a fairway bunker and then into an unplayable lie in desert waste behind the green to go three down.
Kuchar sealed victory by sinking a five-foot birdie putt at the 15th where Day missed his attempt from 22 feet.
"It's fun to continue to survive," said U.S. Ryder Cup player Kuchar, a four-times PGA Tour champion who is seeking his first WGC crown. "I'm so excited to make the championship match. It's a thrill."