He made his debut for Stan Mortensen's men midway through the 1968–69 season, in a single-goal defeat at Portsmouth on 14 December.
He made a further seven appearances before the end of the season, scoring once — a penalty in a 2–1 victory at Crystal Palace on 25 January.
This went part way to helping the Seasiders win promotion to Division One after securing second place at the end of the season.
He made thirteen League appearances in 1970–71 as the club returned whence they had come twelve months earlier, after finishing bottom of the table.
In 1971–72, during which he was loaned out to Halifax Town, Johnston made three substitute appearances, the last of these in a 4–2 victory over Burnley at Bloomfield Road on 1 April.