Mickelson produced a dazzling display of shot-making and nerveless putting in a flawless six-under-par 66 on a day when Rory McIlroy relived his worst nightmare and Tiger Woods faded out of contention.
The left-hander, already a three-time Masters champion, entered the clubhouse at eight-under for the tournament and breathing down the neck of Hanson, who birdied the last two holes for a 65, the best round of the tournament.
"I love it here and I love nothing more than being in the last group on Sunday at the Masters. It's the greatest thing in professional golf," said Mickelson.
South Africa's Louis Oosthuizen, the runaway winner of the 2010 Open, put himself in the frame for a second Major with a 69 to be third at seven-under while American Bubba Watson was a further shot back after a 70.
Saturday is traditionally known as 'Moving Day' and, more than any other tournament, the intense pressure of the Masters brings out the best and worst in every golfer.
While the leaders were thriving in the pressure-cooker atmosphere, McIlroy and Woods both walked off the 18th dejected and frustrated.
McIlroy began the day in a tie for third but for the second consecutive year at Augusta National the Northern Irishman's game collapsed and he tumbled down the leaderboard.
Eight different players, including Lee Westwood, Matt Kuchar and Henrik Stenson, had a share of the lead at various stages during the round, but McIlroy was never in the hunt.
After starting the day one shot behind overnight leaders Fred Couples and Jason Dufner, McIlroy shot a five-over-par 77, that included two double-bogeys, to end at one over.
He took 42 shots for the front nine, rekindling memories of last year when he held a four-shot lead at the start of the final day but closed with an 80.
"Seems like every year I come here I throw a bad nine holes out there," McIlroy said.
“I think if I can finish top 10 it would be a really good result. So if I can go out there tomorrow and shoot something in the mid 60s and walk away from here with a top 10, I'd be very pleased."
A frustrated Woods failed to make up any ground on the leaders and was in danger of his worst finish at the Masters since he turned professional.
Needing to shoot a low score to get back into contention, the four-times champion could only manage an even-par 72 to remain at three over.
He made a bright start, with birdies at the third and fourth holes, as the crowd let out a mighty roar that echoed through the pine trees but failed to make another birdie all day as his emotions started to boil over.
On the 13th tee, he slammed his club into the ground after hooking his drive into the banks of Rae's Creek. On the previous day, he kicked one of his irons after missing a green, but said he was sorry for his petulant behaviour.
“Certainly I'm frustrated at times and I apologise if I offended anybody by that," Woods said.
Defending champion Charl Schwartzel also ended his chance of winning another green jacket after the South African shot a 75 to end the day at six over while England's world number one Luke Donald was a further shot back after also signing for a 75.
Mickelson started the day at two-under. He parred the first nine holes before making his move on the back nine, which he covered in just 30 shots, featuring four birdies and an eagle.
Hanson bogeyed the opening hole but did not make another mistake all day, reeling off eight birdies to lead a Major for the first time.
"It's a new situation to me. I've been up on the leaderboard a few times, but I've never led in anything like this," said the Swede.
"I know it's going to be a tough night ... I probably won't be watching a lot of Golf Channel."