Putnam went even better, wielding a hot putter for a 64 to join Johnson at 15-under 195, the duo five shots clear of Stewart Cink (64).
"I feel my game is really good, really solid", said Pan, a former University of Washington star now in his second year on the Tour.
"I mean winning this week, I think is bigger confidence booster than being number one in the world", Johnson added.
Dustin Johnson waves to fans after making eagle on the 18th to win the St Jude Classic on the US PGA Tour.
Cook's 5 under on Sunday tied him with Phil Mickelson and Tim Herron for the best scores of the final round and moved him up the leaderboard into a tie for sixth.
Johnson, who would reclaim the world's top ranking from Justin Thomas with a win this week, started his round on the back nine and birdied the 10th before back-to-back bogeys at Nos. It was his second win of the season and first since the Tournament of Champions.
Dustin Johnson made eagle at the last to clinch the St. Jude Classic golf tournament. He wound up three-putting for double bogey.
"I was a little flat starting the day - I couldn't really get anything going", Johnson explained.
Johnson shot 7-under 63 for his lowest round this year and grabbed a one-stroke lead Friday after 36 holes at the St. Jude Classic. "Normally you have to drive it really well to succeed here but my iron shots have been good when I've been in the fairway and I've managed myself out of the rough really well".
"I know what it takes", he said.
Yokomine, the victor of 23 events on the Japan LPGA Tour, flirted with a 59 but parred the par-5 18th for a 61 to tie the course record.
Putnam was bogey free in his round of 64 and although a previous member of the PGA Tour in 2015 he was required to return to the Web.Com Tour and now appears to be finding his feet at this level.
Mickelson headed a group of six golfers finishing in 12th at 6 under.
Putnam, a two-time victor on the Web.com Tour, had only one bogey through his first three rounds. And from that moment on, Johnson's second title of 2018 was never in doubt.