On-loan West Ham forward Grady Diangana scored a quickfire brace as West Brom ended Luton's lengthy unbeaten home run in the league with a 2-1 victory at Kenilworth Road.
The youngster was introduced by boss Slaven Bilic at the start of the second half with his side trailing 1-0 and netted twice inside six minutes to earn the Baggies a hard-fought victory.
Cornick was denied a second by the alert Johnstone, who was out quickly to smother from close range, with the rebound bundled behind.
After a poor first half, the Baggies were level just three minutes into the second.
Diangana netted with virtually his first touch, arrowing a drive past the sprawling Sluga and into the bottom corner from 25 yards.
He then made it 2-1 after 51 minutes, instinctively diving to head home a deflected shot from close range to stun the home side, who had been well worth their lead from the first period.
Town substitute Kazenga LuaLua had a go from 20 yards, his daisy-cutter forcing Johnstone to palm the ball behind for a corner, while Pelly Ruddock just failed to reach Dan Potts' enticing cross at full stretch.
The visitors brought on striker Charlie Austin and he had an opportunity straight away, scuffing a shot wide with his left foot.
The £4million signing from Southampton then had an even better chance, set up after some wonderful work from Diangana in beating two players, but crashed his close-range shot against a post.
Austin tried again from further out, hammering over the crossbar, but the Baggies had done enough to end Town's unbeaten run in front of their home fans which stretched back to March 10 2018.