Following months of speculation, and the near-certainty most recently Russell would replace Valtteri Bottas, the team has announced the signing of the 23-year-old Briton on what has been described as a long-term contract.
One of the most promising drivers on the grid now forms a potent partnership with seven-time champion Hamilton as F1 heads into its new era from 2022 given the raft of aerodynamic changes for the cars.
After three years serving his apprenticeship in F1 with Williams, a delighted Russell said: "It's a special day for me personally and professionally, but also a day of mixed emotions.
"I'm excited and humbled to be joining Mercedes next year, which is a huge career step, but it also means I'll be saying goodbye to my team-mates and friends at Williams.
"It has been an honour working alongside every member of the team, and an honour to represent the Williams name in F1.
"Since I joined in 2019, we have worked tirelessly to push each other forward and bring the team back up the grid where it belongs.
"We've battled for every qualifying position, every point, and every tenth of a second.
"No matter how tough it's been, nobody has ever given up, and that has inspired me every day.
"I've loved every moment in what I'd describe as a true heart and soul racing team, and I'll be pushing harder than ever to make sure we end our story in the best possible way.
"Looking ahead to next season, I'd be lying if I said I wasn't absolutely buzzing. It's a huge opportunity and one I want to grab with both hands.
"But I'm under no illusions as to the scale of the challenge; it's going to be a steep learning curve.
"Valtteri has set a high bar, consistently delivering week in and week out, scoring wins, pole positions and helping win multiple championship titles.
"My target must be to reward the trust that Toto, the team, and the board have placed in me by ensuring I play my part in continuing that success and I want to do my new team-mates proud.
"Of course, one of those new team-mates is, in my opinion, the greatest driver of all time.
"I've looked up to Lewis since I was in go-karts and the opportunity to learn from someone who has become a role model both on and off track can only benefit me as a driver, a professional, and a human being.
"For now, though, I have nine more races as a Williams driver, and I want to make sure they are the best nine of my time with the team.
"Then, and only then, can I turn my attention to 2022. A huge thank you to Williams, to Mercedes and to everyone who has supported me in getting to where I am today. I couldn't have done it without every single one of you."
Welcoming Russell to the team as a full-time driver following his years in the junior programme, Mercedes team principal Toto Wolff said: "Looking forward to 2022, we are very happy to confirm that George will have the opportunity to take the next step in his career and join Mercedes.
"He has been a winner in every racing category - and the past three seasons with Williams have given us a taste of what the future could hold for him in F1.
"Now, it is our challenge together to help him continue learning within our environment and alongside Lewis, the greatest F1 driver of all time.
"I am confident that as their relationship grows, they will form a strong team and deliver for Mercedes on and off the track in the years ahead.
"It's a weight off our shoulders to have our plans for 2022 clear and announced; but now, our focus returns to the final nine races of this season and putting everything into our challenge for this year's world championships."
Mercedes curtain comes down on Hamilton-Bottas era
Mercedes' decision brings to an end Hamilton's five-year partnership with Bottas, who has joined Alfa Romeo as a replacement for fellow Finn Kimi Raikkonen after the 41-year-old recently announced his decision to retire at the end of the year.
Hamilton and Bottas have helped Mercedes win the last four constructors' titles, with Wolff adding: "This hasn't been an easy process or a straightforward decision for us.
"Valtteri has done a fantastic job over the past five seasons and he has made an essential contribution to our success and to our growth.
"Together with Lewis, he has built a benchmark partnership between two team-mates in the sport, and that has been a valuable weapon in our championship battles and pushed us to achieve unprecedented success.
"He would absolutely have deserved to stay with the team, and I am pleased that he has been able to choose an exciting challenge with Alfa next year to continue his career at the top level of the sport.
"When the time comes, he will leave us with huge goodwill from every single member of the team, and he will forever be part of the Mercedes family
What do you think?
Congrats to Russel, well deserved and should have come sooner.
Very interesting to see how next year will go, when Russel will hopefully fight for his place on the grid instead of being 'just the 2nd driver'
GPFans is a multi-platform, multi-language brand dedicated to Formula One coverage. We bring you all the ins and outs of the sport, 24/7, everything from up-to-the-minute news and features to the latest viral stories and clips.
We believe that a new generation of exciting, outspoken drivers will make F1 more popular than ever before, and we want to give our users access to as much of their heroes as possible, on and off the track. From Lewis Hamilton to Max Verstappen, Daniel Ricciardo to Sebastian Vettel, we provide in-depth analysis of every every Grand Prix in the season, from Australia to Abu Dhabi.
With Formula One under the new ownership of Liberty Media, how the sport is being covered is evolving, and GPFans will look to be at the heart of this progression into new media, as one of the fastest-growing sites covering the king of motorsports.