Johnston is also the youngest ever champion of BBC2’s prestigious cooking competition aged just 21 at the time of filming.
Owner of the Michelin-starred pub in Maidenhead, Berkshire, and number 45 of this year's Top 50 list, Nick Parkinson highlighted how proud he was of Johnston.
He said: “From the moment Johnston first worked for me, he has always stood out as an exceptional chef and testament to that has been his rapid rise through the ranks in the kitchen.
“We are all so very proud of him, winning the title is totally deserved and it couldn’t happen to a nicer lad. Whatever his future holds, he will always be a Royal Oak chef at heart.”
Johnston fought off stiff competition from Louisa Ellis and Steven Lickley in the final episode to show the judges how far he could push himself.
Challenges in the competition included creating and delivering a Michelin-standard dish for 23 gastronomical giants from around the world, travelling to the south of France to learn in two Michelin-starred kitchens of the fourth best restaurant in the world – Mirazur from Argentinian chef Mauro Colegreco.
The ultimate challenge for the final three chefs was to create the best three-course meal of their lives for judges Marcus Wareing, Monica Galetti and Gregg Wallace in three hours.
Johnston’s winning menu started with torched mackerel and mackerel tartare with salt-baked beetroot, horseradish milk gel, beetroot pickled onions, garnished with beet leaves and a sorrel and dill-iced granita.
His main course was a roasted squab pigeon with thyme-infused pomme anna, a ras el hanout-spiced pigeon leg pastille, charred and roasted thyme onions with a nasturtium oil, radiccio leaves and a red pepper and tomato ketchup, finished with pigeon sauce.
The chef’s dessert was sauterne and yoghurt mousse, basil marshmallow, bergamot curd and olive oil crumble, finished with frozen lemon cells and verbena frozen rocks.
During the tasting of his winning menu, judge Galetti said of Johnston’s dessert: “This speaks volumes about where you started and where you are now as a chef.
“You’ve always given us delicious, beautiful food but it is on another level now. Where are you going to be [in] five or 10 years from now?”
Wareing added: “That is just stunning. It is just perfect. I am speechless.”
Wallace told Johnston he was “a class act”.
Johnston’s reaction to the win was: “Wow. To be the last man standing is unbelievable. I feel like I am dreaming – it’s a dream come true.
“I’ve loved absolutely every minute of it. My parents will be very proud.”
Judge Marcus Wareing added: “You don’t come across cooks like this very often. We’ve just found a star of the future. What a fabulous talent – 21 years old, amazing.”
Fellow judge Monica Galetti added: “Not only is he good in the savoury section, but he has produced some out-of-this-world desserts as well.”
Judge Gregg Wallace said: “The best chefs in the world learn and create their own style, and that is what I have watched Johnston do in this competition – dish, after dish, after dish.”
Johnston trained at a two-AA rosette restaurant Boulters in Maidenhead, Berkshire, after he finished school under chef Daniel Woodhouse. While working there he achieved NVQ levels 2 and 3 in professional cookery from Henley College.
Johnston outlined his future plans, which included heading more towards fine dining and continuing to learn.
He said: “As the next step in my career, I would love to focus more on fine dining and push myself to learn more while I can.
“My ambitions, longer term, are to learn as much as I can still while I am young, and hopefully when the time is right, in years to come, step up to my own head chef role.”