The goalkeeper, who turned 28 yesterday, received his first call-up to the senior side last week.
Johnstone’s impressive performances for Albion this season had led to calls in the media for boss Gareth Southgate to name him in his squad for a series of World Cup qualifiers.
And the goalkeeper admits he was aware of those reports – with the speculation making it even tougher to keep the news from his team-mates when he got a text saying he had been included.
“There was a lot of talk,” Johnstone said when asked if he was aware of the speculation surrounding his call-up.
“And it felt like the longest few days ever leading up to Thursday when the squad was announced
“I’d seen what the media was saying.
“It’s great to see but until you get that call and that confirmation – you know what football is like, anything can happen. It was a long wait until Thursday when that confirmation came.”
Asked how he found out, Johnstone continued: “I was at the training ground at West Brom on the Thursday.
“I was out training and came into a very good text on my phone saying I was in the England squad.
“I was buzzing, it’s such a proud moment for me and my family.
“I was so happy but it was tough because you couldn’t tell anyone. You have to wait until 2pm when it is announced.
“People were asking me, they were saying ‘you must know now’.
“That felt like the longest wait ever leading up to 2pm.”
Johnstone and Albion’s fortunes this season have been somewhat contrasting with the goalkeeper receiving widespread praise for his performances.
The Baggies, though, look highly likely to be relegated from the Premier League with the team rooted in the bottom three.
But while results have been disappointing, Johnstone has still enjoyed the 2020/21 campaign.
“I have been busy this year – it’s totally different to last season when we were top of the Championship and I didn’t have much to do,” the goalkeeper added.
“This year I have been really busy but I have enjoyed it.
“It sounds daft because we are near the bottom, but I have enjoyed making saves and playing against top teams and top players.
“And I have also enjoyed being able to show people what I can do.
“That’s the big thing, I have been able to show people I can make saves and keep West Brom in a game.
“I want to make saves against world-class players.
“Yes, it’s been tough and frustrating at times but that is football.
“I go out there to do my best every game and that’s all anyone can ask for.”