A regular performer for England through all of the age groups, the Manchester United goalkeeper was named in Peter Taylor's squad for the U20 World Cup in Turkey during 2013.
Preston-born Sam had already been capped for his country at U16, U17 and U19 level in the previous few years so has accumulated plenty of experience on the international stage.
He was part of the England U17 group which became European Champions in May 2010, when he started the Finals in Liechtenstein as Head Coach John Peacock's first choice goalkeeper.
However, when he was given a rest in the third group game after the first two victories, he couldn't reclaim his place from Jack Butland in the latter stages. That is something which Peacock has since gone on record as saying was one of the toughest decisions he's ever had to make as Head Coach.
He was then the England U19 goalkeeper during the 2011-12 campaign, which saw Noel Blake's Young Lions reach the Euro Championship semi-finals in Estonia and thus qualify for the U20 World Cup.
A tall and athletic goalkeeper, who is also adept with his kicking ability, Johnstone is highly regarded at Old Trafford too and has appeared on the bench for the United first team.