After a terribly disappointing opening half-hour stalemate involving two teams who have drawn 20 games between them this season, the game suddenly erupted into life on 34 minutes with the first of Johnston's quickfire double.
Winger Tom Fellows got to the right byeline, curled over a lovely hanging right-foot cross and Johnston, only recalled after a hamstring injury to Karlan Grant, arrived unmarked on the edge of the six-yard box to head home.
It was his first goal in 25 Albion appearances since 6 April - yet his next one arrived only nine minutes later.
Having picked up the ball in space on the left, he cut inside, picked his way into the City penalty area and then curled a right-foot beauty inside the far-right upright - a welcome reminder of three of last season's quality strikes in a Baggies shirt.