Thomas, the 2018 Tour de France winner in the former guise of Team Sky, finished on the Grand Tour podium three other times.
He also helped Chris Froome and Sir Bradley Wiggins win the prestigious three-week race around France as he dominated the event, winning it seven times between 2012 and 2019.
Brailsford, who returned to a more hands-on role at Ineos after leaving his partnership earlier this year to take up a post at Manchester United, will be hoping Thomas can help revive their fortunes.
“He knows what to do in the process, how to deal with the ups and downs of a specific game and now his willingness to share that and mentor others to do the same is a huge asset to the team,” the 61-year-old said.
“The fact that he is very humble and has always maintained a great sense of humor are also great qualities to bring to his new role.”
Thomas, a former Whitchurch High School student, was also successful on the track, winning Olympic gold in Beijing, China in 2008 and London in 2012.
He also had success playing for Wales at the Commonwealth Games.
Thomas’ career began as a child in his home town of Cardiff and culminated with the conclusion of the 2025 UK tour in September.
Ineos said Thomas would “work closely with Sir Dave Brailsford, Dr Scott Drower – and the rest of the senior management team, providing key input into race strategy, rider recruitment, development and race preparation”.
The team statement said: “His appointment reflects the team’s long-term commitment to utilizing his vast knowledge of the game and what it takes to win, for the benefit of his fellow Grenadiers.”
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