The 28-year-old earned a call-up to the senior side for the first time last week and is now hoping to be involved when the Three Lions face San Marino in a World Cup qualifier tonight.
Johnstone is no stranger to international football with the keeper part of the England under-17 side that won the European Championships in 2010. He also won caps for England Under-19s and 20s.
Nevertheless, it’s been seven years since he was last involved with the Three Lions. But during that time he’s always believed he could break into the senior side one day.
“I never feared the call wouldn’t come,” Johnstone said. “One thing I knew was that I needed to get into the Premier League to have a chance of being called up into the England squad.
“At Albion we fell short one year by losing in the play-offs. Then it finally happened. And now I’m in my first season in the Premier League, I have had loads to do, I’ve been busy, and I’ve got the call-up. It’s a dream come true.
“But I never shut the door, I would think about it all the time.
“I’d watch the FA TV videos of the goalkeepers training and was desperate to be a part of it.
“But I knew deep down I had to be in the Premier League and playing well to get the call-up.”
Johnstone previously worked with Southgate at youth level.
And he revealed the boss had some words of encouragement for him when he reported to St George’s Park for training on Monday.
“Be yourself, train hard and work hard,” Johnstone said when asked what Southgate has said to him.
“He knows I know a lot of the lads and a lot of the staff here.
“But the main thing was be yourself and enjoy it.”