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 4th June 2021, Johnston signed a three-year deal with Bolton Wanderers. He made his competitive debut for the club in a draw against Milton Keynes.

After becoming a mainstay in the Wanderers back line, with the Whites winning the EFL Trophy and finishing in 5th place in Sky Bet League One, George signed a new deal running until 2026.