But while the duo are friends having come through the youth ranks at Manchester United together, Johnstone says he is going to do everything he can to try and force his way above both Henderson and Nick Pope in the pecking order.
Following a string of outstanding performances for the Baggies this season, Johnstone earned a first call-up to the senior England squad last week.
He has been joined by fellow shot-stoppers Henderson and Pope for the World Cup qualifiers against San Marino, Albania and Poland.
But while all three goalkeepers get on well, Johnstone says ultimately they are there to do a job.
“Dean (Henderson) is a little bit younger than me,” Johnstone said. “We were both at Man United together, we know each other.
“It’s obviously been a while since we last worked together.
“But it was nice to see him and when I got selected he sent me a text, which was nice because we hadn’t really been in contract for a few years.
“It’s been really good to see him again.
“I am new to the senior squad but they (Henderson and Pope) are more experienced than me in this set-up.
“I’m here to work hard and to push both of them. And ultimately make it difficult.
“I want to do well, I want to keep coming back so it’s up to me to do well in training and show what I can do.
“I’ve worked with both of them over the last two days, they are both great lads and great goalkeepers.
“We push each other, it’s good competition.”
As well as receiving a text from Henderson, Manchester United’s David de Gea also got in touch with Johnstone to congratulate him on his call-up to the national side.
“He text me when I got the call-up and said he was really happy and proud,” the 28-year-old revealed.
“It was really nice to hear from someone like that, I trained alongside him for a number of years and learnt a lot from him.
“He said I deserve it, good luck and to give it my all.”
Asked what in particular he learnt from De Gea, Johnstone said: “He was young but he was playing for Manchester United and Spain.
“To be around him every day was special.
“Realising how calm he is was the big thing.
“He is one of the best.
“It was great to be around him and learn from him and then I had to go and make my own path and my own career.”
Johnstone also knows Jesse Lingard and Luke Shaw from his time at Manchester United – with both of those players having been recalled to the England squad for the up coming games.
“I grew up with Jesse from when I was 10-years-old,” added the goalkeeper, who has previously represented England at youth level.
“Luke, I was with him at Manchester United for three or four years as well.
“It’s been nice to see them again.
“We always kept in touch.
“But to be honest with this squad I know a lot of the lads just from the youth set-up over the years.
“There are loads of faces that I saw that were nice to see and nice to work with again.”