The 24-year-old defender signed a five-year deal in joining from Montreal and is fresh from representing his country at the tournament in Qatar, where he played against Hoops full-back Josip Juranovic in a 4-1 defeat to Croatia.
Johnston's arrival means Celtic now have three right-backs on the books, although Juranovic has been linked with a big-money move away from the club in January after some terrific performances in Qatar. Scotland defender Anthony Ralston is also standing in Johnston’s way, however, the new recruit stressed he is ready to challenge for a first-team spot – whether Juranovic stays or goes.
“I did actually talk to (Juranovic) after (the game) and said hello," Johnston said. “He was really good about coming to find me after a big 4-1 win for them. He is a great player.
“Nothing is going to be given (at Celtic), everything will be earned. That is something all aspiring footballers want, an opportunity, and that is something the coaching staff has been very strong about, that we are trying to play a certain way and we want to bring in some young, talented players that have an edge to them.
“I fit that mould but the other two right-backs have that as well but I am here to fight for my place and hopefully earn some minutes.
“It is very attacking at Celtic. It is exactly how I like to play the game. It is a good fit for me. It is a match made in heaven for me, so it was a pretty easy decision.”
Johnston revealed Canada head coach John Herdman has likened him to Chelsea and Spain defender Cesar Azpilicueta.
“The ability to play in a back three, a back four, a back five, that flexibility that he brings, super-consistent,” he added.
“Definitely I have a lot of physicality to me. I just love to defend and that is something that everyone will see.
“For me, putting in a big tackle is just as good as getting on the end of a cross. That is what you will see from me. I live to defend.”
Johnston revealed he has been hit with an “outpouring of love” from Celtic fans since joining a club that he grew up watching on the other side of the pond.
“My dad’s side is English and Scottish and my mum was born in Northern Ireland. Growing up in a UK household, football was our main sport. We played some hockey in the winter which brought a bit of grit. You can’t not do that when living in Canada, you have to play some hockey.
“So my entire childhood was waking up watching the Premier League and Champions League and of course, seeing Celtic in some of those big nights.
“For me it’s always been one of those clubs I’ve been drawn to. I always looked at it like this, if there is a full slate of games on in the Champions League and Celtic are playing at home, I want to watch Celtic, it doesn’t matter who they are playing.
“It’s just to see the atmosphere because you can tell how much more it means to the fanbase and what those European nights are like.
“We have a couple of players with the national team and they said the league is a blast and Celtic are the cream of the crop. I’ve only heard glowing reviews and it’s cool to live out that childhood fantasy.”
Asked about his early days as a Celt, Johnston added: “It has been hectic. I tweeted about it a couple of times but it is no joke, the Celtic fans almost broke my phone, just the outpouring of love has been pretty crazy. That, doubled up with participating in a World Cup at the same time, it is a crazy time in my life. I am super-excited to get here.
“Everyone has been so warm and welcoming. Even just walking around the town centre people have been coming up and saying, ‘congratulations, you have signed for the biggest club in the world’. It has been a unique and special welcome.”