It meant that Sainz had to spend much of 2024 trying to find a new team for next season, with Mercedes, Red Bull and Sauber/Audi all seemingly in the hunt to acquire the Spaniard's services.
Sainz has had his best ever season in 2024 in terms of race wins, claiming two, the first time he has ever secured more than one grand prix victory in a season.
He will head to Williams full of confidence, but also with some trepidation about how high up the grid he will be competing with his new team.
In recent days, reports have emerged about an apparent release clause in Sainz's contract that would allow him to leave Williams, if Red Bull come calling in an attempt to replace the out-of-form Sergio Perez. Williams denied the reports in an official statement to GPFans.
Perez has scored just 12 points from his last six outings, and Red Bull chief Helmut Marko has regularly aired his frustrations at the Mexican's inability to get close to team-mate Max Verstappen and help the team's constructors' championship quest.
Now, team principal Horner has provided an update on the rumours linking Sainz to a move to Red Bull.
"I mean, there’s always a lot of speculation in the Formula 1 paddock," Horner told talkSPORT. "Carlos Sainz is a Williams driver next year. He’s a great driver, he had a big win in Mexico recently and I think he’ll be very good for that team.
"Carlos is committed to Williams and he is not part of our plans for next year," the Brit comprehensively concluded.