Thomas was prominent at the front of the yellow jersey group that rode tempo on the Col de Joux Plane and then took no risks on the wet and narrow descent into the alpine town.
Froome finished the penultimate stage of the Tour with a smile on his face, surrounded by a full strength team that will embark on a processional run into Paris on Sunday.
“I rode the climb and we told Froomey to sit behind me, so we had three guys behind him in case anything happened,” a soaked Thomas said past the finish line. “I just went down nice and steady. We couldn’t have done any better.”
Highlights of stage 20 of the Tour de France
Thomas himself will finish the Tour 15th overall, equalling his career best result from the 2015 edition that inspired him to skip most of the spring classics this year and try and improve his placing here. That plan was shelved when he crashed in the opening week and injured ribs though.
The 29-year-old has supported Froome to all of his Tour victories to date and said the composition of the squad this year, which included newly recruited and familiar teammates, was a point of difference.
“I think the strength in depth has been massive in the team,” Thomas said. “We’ve got a lot of good climbers here, probably the most we’ve had than any of the Tours. It’s the first time we’ve still got nine riders in the race so it’s been a great three weeks.
“The closer you get to the finish the more confident everyone gets.”
Froome will enter Sunday’s 21st and final stage with a four minute and five second advantage over Romain Bardet (Ag2r La Mondiale) with Movistar’s Nairo Quintana, the Colombian that didn’t as anticipated really take it to the Briton, a further 16 seconds in arrears.