The South Korean finished a remarkable 15 under on his debut in 2020 but was beaten into second place by a record-breaking performance from Dustin Johnson as Covid-19 forced the Masters from its traditional April spot.
That remains his best finish on the DP World Tour but the 24-year-old is full of belief he can become the second Asian winner of the Green Jacket 12 months after Japan's Hideki Matsuyama made history down Magnolia Lane.
Cameron Smith - who finished second at this tournament alongside Im in November 2020 - was a shot behind at four under, one ahead of Johnson, 2016 champion Danny Willett, World Number One Scottie Scheffler and Chile's Joaquin Niemann.
Americans Patrick Cantlay and Jason Kokrak were then at two under alongside Canadian Corey Conners, one clear of 15-time Major winner Tiger Woods, who fired a 71 in his first competitive round since suffering severe leg injuries in a car accident 14 months ago.
Im is a two-time winner on the US PGA Tour, where he was Rookie of the Year in 2019, the same season he made his Presidents Cup debut.
But he admitted it was his performance here 17 months ago that was fuelling his fire this week.
"I have great memories from 2020," he said. "It was my first ever Masters and to finish runner-up at my first Masters was great.
"That memory kind of lingers around and I still remember that feeling and it gives me confidence when I come back here that I can finish strong or possibly win it here someday."
He added: "I don't want to think about overachieving anything at this moment. I still have a lot of days of play and there are a lot of talented players out here at the Masters.
I still remember that feeling and it gives me confidence when I come back here that I can finish strong or possibly win it here someday
"I think with the rain passing by, the greens are going to get firmer, the fairways are going to get firmer, so I just have to be prepared to do the same thing I did today."
Im made a flying start, putting his second to seven feet at the first, two-putting the par-five second and holing a 17-footer at the third to hit the front.
That handed him the lead but he would soon find himself trailing a flying Smith.
The Australian had made a double-bogey on the first but a chip-in on the fifth kick-started his round and he added gains on the sixth, eighth, ninth, 12th and 14th to hit the front.
He made it a hat-trick of gains on the 15th and 16th to lead by two but he bookended his round with another double-bogey to leave the door open to the rest.
Im had put an approach to 12 feet at the seventh for a fourth birdie of the day but he made back-to-back bogeys on the tenth and 11th before putting his second to 12 feet at the 13th to make an eagle and tie the lead.
He then got up and down at the par-five 15th after laying up and he was the solo leader.
Johnson and Scheffler had both been bogey-free at four under before dropping shots on the 17th and 18th respectively, while Niemann made a spectacular hole-out eagle from 105 yards on the ninth in his 69.
Willett took advantage of the four par-fives and birdied the ninth with a stunning 39-foot putt that broke a long way from right to left, also making bogeys on the first and sixth.