The announcement on Monday came after the Williams academy driver secured a Formula One super-licence in Sunday's season-ending Formula Two race in Abu Dhabi.

Florida-born Sargeant, who has been living in London for the past five years, finished fourth overall in the feeder series and was rookie of the year.

He replaces Canadian Nicholas Latifi (27) alongside British-born Thai Alex Albon (26) at Williams - the former champion team who finished the season last in the constructors' standings.

"I feel like I've put in as much work as anyone else to reach this point and I just have to prepare the best I possibly can to be the best driver I can possibly be next year," Sargeant told reporters on a video call from Abu Dhabi.

"Hopefully, that's good enough to stay in F1 for a long time."

"I don't think it's any extra pressure," he said of being an American in a sport with three US races and a fast-growing fanbase in his homeland thanks to the Netflix Drive to Survive docu-series.

"I have high expectations for myself as it is."

Williams had said at the US Grand Prix in Austin last October, where Sargeant took part in first practice, that he would be racing for them next year providing he secured the licence.

"We believe he is absolutely ready to get into Formula One," team boss Jost Capito said then.

Formula One has for years been eager to have a US driver, with Red Bull recently thwarted in a bid to sign Colton Herta (22) for their AlphaTauri team because the IndyCar racer lacked a super-licence.

Sargeant, who moved to Europe as a youngster and has progressed through the various junior series, said he liked to think his nationality was just a "happy coincidence" in his getting to Formula One.

He said waking up on Monday as a Formula One driver felt very natural and he was at home in the team.

"I feel like I've had a very good junior formula career and I'm just looking forward to closing that chapter and moving on to the next," he said, adding that he had yet to decide his driver number.

Sargeant finished third in the 2020 Formula Three championship, four points behind Australian winner Oscar Piastri (21) who will be making his race debut with McLaren next season.

"There was never any friction. We've been friends since karting," he said. "Everything he's done I'm very happy for him and I hope we can both be successful coming into F1."