Johnston came through the Liverpool Academy, before moving to Feyenoord on a permanent deal. This was followed by a loan move to Wigan Athletic for the remainder of the 2021-22 season. Johnston has also represented Scotland at the U20s and the U21s.
On 1 July 2021, Johnston signed a three-year deal with Bolton Wanderers. He made his competitive debut for the club in a draw against Milton Keynes.
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