Head coach Chris Chester told his side they had to win this game at all costs even if it was ugly.
They did get the victory – only a second in nine matches – but also did so in some style with their biggest ever Super League success over Hull KR.
Winger Johnstone claimed his treble, just as he did in the reverse fixture on the opening day of the campaign, while recalled full-back Scott Grix (2), Ben Jones-Bishop, Matty Ashurst, Tyler Randell and Anthony England all scored tries.
In the hot temperatures, Trinity initially looked sluggish during the first period yet somehow found themselves 18-12 ahead despite KR creating a raft of chances.
However, they were a far different entity in the second half when, with stand-off Jacob Miller becoming increasingly influential, they were able to pull away from their lowly rivals.
Liam Finn, restored to the side after being dropped against Castleford, slotted a 44th minute penalty and when KR infringed again it was Miller’s astute pass that sent Grix over for the first of his brace.
Trailing 26-12, Rovers were still in the contest on 52 minutes so they were furious when referee James Child missed Jones-Bishop’s blatant push into the back of Chris Clarkson when challenging for Maurice Blair’s high kick.
Jones-Bishop duly picked up the scraps and raced 80m to score the try that killed off any hopes of a KR victory.
Miller’s kick found Johnstone for his second before Grix dummied through from 20m.
Kieran Moss got a consolation for the visitors in the 68th minute but England benefited from another Jones-Bishop break and then Johnstone rounded things off.
Trinity opened the scoring when Matty Ashurst supported Jacob Miller’s quality stepping break in the eighth minute, Finn slotting the first of his nine goals.
Soon after, they doubled their advantage. Rovers prop Robbie Mulhern charged close but Danny McGuire’s grubber only found Miller who sprinted 80m to the other end. He didn’t quite have the speed to finish off but Johnstone popped up on his shoulder to finish off.
Tim Sheens’ side finally got some rewards with a thrilling try from Clarkson.
They immediately added their second in the next set, too, with McGuire’s quality inside pass unleashing Adam Quinlan down the middle.
The Australian full-back finished well and Shaw converted both to leave it 12-12 after 21 minutes.
But, with a rare Trinity attack, Tyler Randell went over from dummy-half exposing some soft defence in the 28th minute and his side never looked back.
Wakefield Trinity: Grix; Jones-Bishop, Lyne, Tupou, Johnstone; Miller, Finn; England, Randell, Hirst, Pauli, Ashurst, Arona. Substitutes: Wood, Batchelor, Fifta, Huby.
Hull KR: Quinlan; Shaw, Blair, Vaivai, Salter; McGuire, Lee; Mulhern, Atkin, Masoe, Clarkson, Tickle, Kavanagh. Substitutes: Smith, Greenwood, Walne, Moss.
Referee: James Child (Dewsbury).