Russell, 23, is in his third Formula One season with Williams and scored the first podium of his career at the controversial Belgian Grand Prix.
His form for the backmarking team this season has been sensational, especially in qualifying, and has made the decision seem inevitable for a while.
As a Mercedes-backed driver, he has been linked with a step up to the senior team for several years.
Russell became part of the Mercedes racing family aged 15, when he gave current boss Toto Wolff a Powerpoint presentation listing all the reasons he deserved financial backing from the company. The presentation, combined with Russell’s impressive results at that time, convinced Wolff he was worth taking a chance on.
In the years since, Russell has vindicated that decision, winning the GP3 (now Formula 3) championship in 2017 and the Formula 2 championship in 2018.
He is considered one of F1’s best young talents and will now have the chance to test himself in equal machinery against Hamilton, who trails Verstappen by just three points in this year’s title fight and could be an eight-time world champion by the time 2022 starts.
Russell has actually raced as a Mercedes once before, as a stand-in for Hamilton following a positive COVID-19 test ahead of last year’s Sakhir Grand Prix. Russell looked set to win that race until a late Mercedes pit-stop blunder relegated him down the order.