The American was on course to shoot just the second 59 in the history of the DP World (European) Tour when eight under par through 13 holes at the Renaissance Club coastal course, east of Edinburgh.
But Thomas, twice a winner of the US PGA Championship, could only par the last five holes.
"I thought the par was 71 and then I looked up and I saw it was 70 and I was like, 'I just need a couple more' (birdies) and I can shoot 59," Thomas said.
"And then, shocking, I made five pars in a row... I definitely wanted a 59 and was even thinking a couple lower, so we all know how that works out when you get ahead of yourself.
"But it's always good to get off to a good start, and even better to get off to a great one. I felt like I was in great control of everything."
Thomas was not the only player to enjoy a low first round in a tournament that also acts as a warm-up event for next week's British Open -- golf's oldest major championship -- which this year takes place at Troon, on Scotland's west coast.
South Korea's Im Sung-jae was just one shot behind, with Ludvig Aberg on six under and defending Scottish Open champion Rory McIlroy one of several players on five under.
This was McIlroy's first tournament since his agonising finish at last month's US Open, where the 35-year-old Northern Irishman missed out on a fifth major title after a dramatic collapse on the closing holes at Pinehurst, North Carolina.
But he made a strong start to his Scottish Open defence with an eagle on the third hole, his 12th of the day, in a round also featuring five birdies.
"I hit a couple of shots over the first few holes that I haven't seen in practice over the last 10 days so it was a bit of a reminder that golf isn't as easy as sometimes I think it is," McIlroy said with a smile.
"But then I really started to feel good. Especially on the front nine, our back nine, I started to hit some good drives, some good iron shots."
He added: "Overall a good start, especially after the scrappy start over the first six or seven holes."
McIlroy held a two-shot lead with five holes to play in the US Open but bogeyed three of the last four, missing from two feet and six inches on the 16th and three feet and nine inches on the last to finish a shot behind Bryson DeChambeau.
"I'm not going to let three or four holes cloud my judgement in terms of how good I'm playing," McIlroy insisted.
