Johnstone has lived in England since he was very young; as a junior, he played for Stanningley Amateur Rugby League Club.

He attended Pudsey Grangefield School and represented the City of Leeds sprinting team at Indoor athletics.

He made his full Super League début for an injury-hit Wakefield Trinity side in their 2015 Round 7 defeat by Wigan.

Johnstone was one of the favourites in 2019 to become Super League's top try-scorer and he appeared well motivated to achieve that accolade until injury struck.

[4] In round 1 of the 2021 Super League season, Johnstone scored two tries and received the man of the match award in Wakefield's 28–22 loss against Leeds.

[9] In round 13 of the 2023 Super League season, Johnstone scored two tries against his former club Wakefield Trinity as Catalans won 36-6.