 Johnston's position of choice was but his speed also saw him play on the , especially in representative teams.An athletic speedster who played Colts for Eastern Suburbs RUFC while studying at the Institute of Technology in Sydney, Johnston signed with St George in 1980 and played for New South Wales in the State of Origin series on eight occasions between 1984–1989.He represented in the centres or at wing for the Blues in all three games of the 1987 State of Origin series plus the 4th 'exhibition' game played in Los Angeles.