He made his debut in Round 3 of the 1980 VFL season against Hawthorn at Princes Park.
A member of Port's 1981 premiership team, Johnston was appointed club captain in 1986.
He missed the 1988 Grand Final win due to suspension but captained Port Adelaide to premierships the next two seasons.
Johnston's performance in the 1989 Grand Final earned him a Jack Oatey Medal, to go with the Port Adelaide 'Best and Fairest' which he also won that year.
In 2000 Johnston was named as first ruckman in Port Adelaide's official Greatest Team, which took into account all players since 1870.