The centre first played schoolboy rugby for Australia 'A' after winning the Associated Independent Colleges competition with St Edmund's College in 2011.
He played Rugby League for the Gold Coast Titans U20s after school, before returning to Rugby Union at the end of 2013, when he joined the Queensland Reds wider training squad and returned to his former club GPS.
Johnson made two Super Rugby appearances for the Reds and was named on the bench a further three times, before joining Glasgow Warriors.
The Australian centre was jointly voted as Player's Player of the Season at the 2017/18 awards dinner, having impressed with a string of consistently high-quality performances in the Warriors midfield.
He was rewarded with a first Scotland cap in the 2019 Guinness Six Nations opener against Italy at BT Murrayfield, scoring tries against Ireland and - in memorable fashion - England in that year's tournament.
Johnson earned the first Player of the Match award of his professional career in October 2021, claiming the prize in a 35-24 victory over the Sharks at Scotstoun.
The centre then captained the club for the first time the following month, leading the team in a 19-18 defeat to Benetton in Treviso.