Johnston played 209 games and kicked 283 goals for the Carlton Football Club (The Blues), in a career spanning 1979–1990.
He was the recipient of the Carlton Best and Fairest award, the John Nicholls Medal (at that time called the Robert Reynolds Trophy), first in 1983 and then in 1986, when he tied with Craig Bradley.
For 1991, Johnston was signed by struggling SANFL club Sturt and was regarded as a key recruit in the hope of improving the Double Blues from only two wins in 1990, but injuries meant he played just three games for the Double Blues before injury ended his playing career and he was sacked ten weeks before the SANFL minor round ended.
He stayed connected with football, however, and for a time coached the Mt Gravatt Vultures in the AFL Queensland South-East Premier League.
[6] Following the breakup of his marriage, Johnston and family moved back to Melbourne; there he found a new love, actress Kate Kendall, and they married on 28 November 2004.