The 2017 PGA Championship victor seized his first triumph since last year's WGC Bridgestone Invitational, defeating Cantlay by three strokes, and claimed the top spot in the US PGA playoff point standings entering next week's season-ending Tour Championship.
"I was really nervous today", said Thomas, who began the day with a six-stroke advantage.
Asked what it had been like heading into the last round with such a big lead, Thomas told Sky Sports: 'It was hard. I was enjoying it though. "I thought if I shot 6-under-par I might have moved on". Thomas rolled in a four-metre birdie putt at the last for a closing 68 and 25-under par overall, clinching his 10 win on tour but his first in a year. The hole allowed Cantlay to cut the lead in half, but Thomas birdied four of the remaining eight holes.
Thomas said: 'I've never slept on a Wednesday lead before, so that's going to be weird.
"It was just one of those freaky days when you get in a zone", Thomas said. "I was hitting it long and straight, and that was fun". I couldn't hit a fairway. "That was a hard start".
After seeing his lead shrink to two shots, Thomas landed his approach two feet from the cup at 11 to set up a birdie and surged to the finish.
"The birdie on 11 was huge", Thomas said. "That propelled me for the rest of the round".
Successfully returning last year from major back surgery, he ended a five-year drought with his two-stroke win at East Lake, the 80th of his PGA Tour career, which was at the time just two shy of Sam Snead's all-time mark. This year, the golfers will be seeded, with Thomas opening the tournament already at 10-under par and No. 2 Patrick Cantlay at 8 under down through the 26th through 30th qualifiers who will start at even par.
"It's not like I've been playing bad. I'm going to have a have another good round and see if we can make some history". I got in my vehicle and left and couldn't get back.
Matsuyama had the lowest round of the day at 9-under 63 to jump into third place (20 under).
American Tony Finau finished fourth on 270, one shot ahead of countryman Brandt Snedeker and Spain's Jon Rahm.
But Finau's 69 was not enough for him to overtake Bryson DeChambeau for the final automatic berth on the United States team for December's Presidents Cup in Australia, qualifying for which ended Sunday.
Thomas birdied the first five holes, answered a bogey at six with a birdie at the par-5 eighth, hit an awesome approach at the par-5 10th to set up a tap-in eagle and sank a 22-foot birdie chip from the rough at the par-5 14th.
But Thomas steadied with clutch play over the next three holes, twice holing putts of around 12 feet to re-establish his authority.
That includes Lucas Glover, who went bogey-double bogey until finishing with a two-putt par from 40 feet to secure his first trip to the Tour Championship in 10 years.
It will be the first official PGA Tour event in Japan.
Adam Scott and Marc Leishman are into the Tour Championship at East Lake in Atlanta next weekend but they will have some serious chasing to do under the tour climax's new format.