Im turned professional in 2015. In 2016 and 2017 he played on the Japan Golf Tour, his best finish being joint runner-up in the 2017 Mynavi ABC Championship. In 2017 he finished 12th in the tour money list and 5th in the scoring average. He played a number of tournaments on the Korean Tour, his best finish being joint runner-up in the 2017 T-up Gswing Mega Open.
In December 2017, Im finished second in the Web.com Tour Q-school, including a third round of 60. He won the first event of the season, The Bahamas Great Exuma Classic, becoming, at 19 years, 292 days, the second youngest winner in Web.com Tour history. Only Jason Day, at 19 years, 238 days, had been a younger winner. Im had three solo runner-up finishes, in The Bahamas Great Abaco Classic, the Knoxville Open and the Pinnacle Bank Championship, and finished the regular season by winning the WinCo Foods Portland Open. He led the regular season money list and was named Web.com Tour Player of the Year.
In the 2018–19 PGA Tour season, Im won the PGA Tour Rookie of the Year. He became just the 13th rookie to qualify for the Tour Championship in the FedEx Cup era, ultimately finishing 19th in the standings. Im led the tour in starts and cuts made , and his 118 rounds were 18 more than the nearest competitor. He had seven top-10s in 2019, highlighted by a T-3 at the Arnold Palmer Invitational, and finished 17th in strokes gained. Im joined Stewart Cink (1996–97) as the only players to be named the Korn Ferry Tour Player of the Year and PGA Tour Rookie of the Year in consecutive seasons.
On 22 September 2019, Im lost the Sanderson Farms Championship in Jackson, Mississippi in a playoff to Sebastián Muñoz.
In December 2019, Im played on the International team at the 2019 Presidents Cup at Royal Melbourne Golf Club in Australia. The U.S. team won 16–14. Im went 3–1–1 and won his Sunday singles match against Gary Woodland.
On 1 March 2020, Im won The Honda Classic at PGA National Resort and Spa in Palm Beach Gardens, Florida, with a final round of 66 and an overall score of −6. He finished one stroke ahead of Mackenzie Hughes, and moved to second place in the FedEx Cup standings.
In November 2020, Im finished tied for second place at the Masters Tournament; five shots behind Dustin Johnson.
