Johnston attended George Watson's College in Edinburgh and played rugby union, captaining the Scotland schoolboys team.
[1] Johnston attracted the attention of football scouts with his performances for the Hutchison Vale boys club.
[1] He signed for Heart of Midlothian in 1977 and made one appearance in a First Division match against Dundee, but was substituted due to injury.
Johnston was part of the Scotland team that won the Grand Slam in the 1984 Five Nations Championship and he scored a try in the win that year against England.
He took on the role of assistant coach for the 1988 Scotland rugby union tour of Zimbabwe, working with Richie Dixon.