As the leader and holder of the race's yellow jersey, tour convention means other riders will not challenge him during the final stage.
Defending champion Chris Froome is also out of contention for the overall victory but leapfrogged Slovenian Primož Roglič, the victor of yesterday's final mountain stage, into third on the general classification standings after he crossed the line today in second.
Thomas will take a lead of 1 minute, 51 seconds into Sunday's final stage to Paris, which is traditionally a procession.
In Froome, Thomas saw innocence, fierce determination and huge talent.
Thomas, known as "G", was a support rider during Froome's title rides but he became Sky's undisputed leader when Froome cracked in the grueling 17th stage through the Pyrenees.
It's no luck that Geraint Thomas has enjoyed such huge success at this year's Tour de France.
Thomas finished third in the 20th stage, 14 seconds behind, but that was more than enough to protect the lead of more than two minutes he had at the start of the day.
Thomas' victory will come as a relief for Sky, especially in the wake of Froome's popularity nose-diving in France after he was cleared of a doping offence months after testing positive for excessive levels of an asthma drug. "I heard I was up and maybe I was pushing it a bit hard on some of those corners", Thomas said.
Tom Dumoulin's Team Sunweb were fifth on the day, but only gave up seven seconds to Sky and Thomas before some much more significant time gaps further down.
"I know people won't believe it, but it was only on Friday night that I started to think about it", he said Saturday (via the Guardian). "I dreamed about this victory for many years and I was close many times before but I didn't really manage to beat the faster guys like Greipel, Kittel and Cavendish".
Thomas was embraced by his wife Sara and Team Sky principal Sir Dave Brailsford at the finish, and appeared to wipe away a tear.
French rider Sylvain Chavanel, riding his record 18th Tour, rode ahead of the pack as the first rider onto the Champs-Elysees. "I'm surprising myself on this Tour but we're not in Paris yet".
"To win on Alpe d'Huez in the yellow jersey".
But with his clothing sponsor based just across the Spanish border in San Sebastian, an emergency call was placed and they whipped him up a new set of the world champion's rainbow stripes in time. He's been a massive part of my Tour victories. But he at least salvaged a podium finish after dropping to fourth overall following Friday's last mountain stage. "It's just overwhelming. I didn't think about it for the whole race, and now suddenly.I've won the Tour, man!"
Fans will be expecting Froome, a six-time Grand Tour champion, to overcome his 13-sec deficit to Roglic and secure a place in the top three.