The 24-year-old has signed a five-year deal and will join up with his new team-mates at the end of next week following his exploits in Qatar. He started all three of Canada's games and impressed against Eden Hazard.
Ange Postecoglou admitted his delight at the signing, the second this month following Yuki Kobayashi, and revealed where he expects Johnston to play.
“I’m delighted,” he told the Celtic website. “He’s someone we identified pretty early on as somebody we think can add to our club. He’s had an interesting career path where he’s had to work very hard to get to where he is, and we’re very happy to conclude the deal and knowing he was going to the World Cup, I think he performed well there as well. He’s really keen to come across and take the next step in his career, and I think he’ll be a great fit in our dressing-room.
“He’s predominantly played on the right side of defence, at centre-back or right-back. He’s very good technically, very good going forward and also a really strong competitor and he doesn’t mind the physical aspects of the game, and as I said, I think he’ll add something to our club.”
Johnston, whose mum is from Northern Ireland, called the move a “perfect fit”, admitting it was a transfer which turned his head.
He told Celtic TV: “When a club like Celtic comes calling, it will turn any footballer’s head, and especially for myself, growing up in a British household, Celtic was one of the clubs that is near and dear to our hearts. And for me, it’s one of the biggest clubs in the world. Just to hear that this was a club that was interested in me, and to see the project the coaching staff have and the progress that this team and this club continues to make in Europe as well, it’s really exciting.
“Ange really was the one who sold me on it. You want to go somewhere that, not only do you feel that you’re a part of something and you feel that you’re wanted, but at the same time, it’s a good sporting fit as well.”