Johnston was just 21-years-old when he triumphed in the BBC show, becoming the youngest ever winner in its ten series.
He left his role as sous chef at The Royal Oak gastropub in Maidenhead earlier this month to take up his new position at the two-Michelin-starred kitchen.
Wareing, who judges the BBC show alongside chef Monica Galetti and Gregg Wallace, was left “speechless” during the tasting of Johnston’s winning menu in the series finale, which aired in December.
The Marcus restaurant tweeted: “We are thrilled to announce that last year’s @MasterChefUK winner, Craig Johnston, has joined our incredible team @MarcusBelgravia as Senior Chef de Partie. We are really excited to have this hugely talented chef join us! Watch this space!”
Speaking after his win in December, Johnston said: “As the next step in my career, I'd love to focus more on fine dining and push myself to learn more whilst I can.
“My ambitions longer term are to learn as much as I can still while I'm young, and hopefully when the time is right in years to come step up to my own head chef role.”