Stoke City stunned their visitors with a brilliant performance in front of the Sky Sports cameras to move them up to third in the Championship table.
The Potters were in again just two minutes later, but Johnstone was able to block Josh Tymon's shot from creeping underneath him and into the net.
The Baggies were determined not to make it a one-sided affair though, and Callum Robinson's three consecutive shots saw Adam Davies called into action on 20 minutes.
Referee Tim Robinson had a huge call to make with five minutes if the first half left, with Darnell Furlong calamitously heading into his own net after Mario Vrancic had knocked Callum Robinson off the ball.
Robinson contentiously adjudged the Bosnian to have fouled his opponent and ruled out the goal, sparing Furlong and Johnstone's blushes.
Into the second period, Vrancic hit the crossbar from a direct free-kick with 50 minutes gone.
Another huge call was made with 20 minutes to go, as Johnstone impeded Stoke's Sam Surridge in the box and a penalty was awarded.
Surridge stepped up, but his poor penalty was saved by the England international. It just did not look like it was City's night.
But with ten minutes to go, star man Powell provided a deserved winner for the hosts by deftly lifting the ball over the hapless Johnstone after Tommy Smith had played him through.
That was that for Stoke as they sealed a vital three points against their close rivals - both geographically and in the race for promotion, and handed them their first league defeat of the season.
It was clear that the Potters deserved their win on the night, and it was certainly no fluke.
Michael O'Neill set his side up similarly to the previous 1-1 draw at Preston, with a 3-4-2-1 formation slightly differing to the 3-4-1-2 that they begun the season with - and it proved to work to an even larger extent against the Baggies.
Valerien Ismaël's direct style was completely ineffective with Jordan Hugill unable to make the ball stick due to being marked by man-mountain Harry Souttar, and inside-forwards Callum Robinson and Karlan Grant snuffed out by the tenacity of Potters' wing-backs Tymon and Smith.
The Baggies boss himself described Stoke as "the best team we've played this season".
There was no switch of tactics by Ismael despite it clearly not working in the first half, and this could be where he and his team come unstuck this season.
Any team chasing promotion must have good home form in order to contend for the top spots.
Take last season for example, where promoted sides Watford, Norwich and Brentford had the best home records in the league. Stoke look to have nailed that so far this season, with five wins and one draw from six games in ST4 so far.
The bet365 Stadium was at the peak of its powers when formerly known as the Britannia Stadium, hence the birth of the famous phrase 'a cold, windy night in Stoke'.
If Michael O'Neill's men can channel some of that fortune of old this season, then anything can happen for this Stoke City team.
Stoke's striker dilemma
Despite having five first team strikers on the books, one slight negative is that Stoke are lacking goals and fitness from their frontmen at the minute.
None of Surridge, Brown, Abdallah Sima or Steven Fletcher have scored in the last five, with all of them scoring just four of Stoke's 15 goals so far in the league.
This hasn't been a problem so far for the Potters, with goals coming from all over the pitch - most notably Nick Powell with five in just seven games.
However as the season goes on, the strikers will certainly need to step up to the plate if Stoke are to dispatch of teams like they have been struggling to in some games recently.
A huge plus for Michael O'Neill is that key man Tyrese Campbell is still to come back from his long-term injury, with the 21 year-old likely to be finally back in the fold after the international break.