Johnston came through the Glasgow and West pathway, playing his school rugby at Kelvinside Academy and previously representing GHA. He featured for Glasgow Warriors U18 during the 2024/25 campaign and was an academy-supported player within the regional programme.
A move to Hartpury followed, with Johnston combining his rugby with a Sports Business Management degree. His performances earned him selection for the Scotland U20 squad for the 2026 Six Nations.
Johnston returned to Scotland towards the end of the season and was selected for the Scottish Thistles at the 2026 Melrose Sevens.
The tournament proved a successful one, with the Thistles defeating Edinburgh Accies 22-10 in the final.
He has now joined Glasgow Hawks ahead of the 2026/27 Premiership campaign.