Johnstone's first came at the end of a half in which Trinity had thrown away a 12-0 lead and stopped Salford going in level at the break.
He scored again before the visitors reduced the arrears through Justin Carney's try.
However a third score to add to Reece Lyne's effort ended Red Devils hopes.
Since losing in front of Chester at Hull, Trinity have found form to beat Huddersfield, Leeds and now Salford, who were in action for the first time since crowd violence marred their victory at Huddersfield.
Jacob Miller and Michael Sio crossed for Wakefield on the back of cheap possession early on but Salford, prompted by Michael Dobson, fought their way back through Gareth O'Brien and Matt Sarsfield tries.
However winger Johnstone's score before the break was a blow from which the Red Devils never quite recovered, and the loss of Carney to an ankle injury was salt in the wound.
The Wildcats have now achieved more Super League wins this season than they managed in 23 games last year.
Wakefield: Jowitt, Jones-Bishop, Arundel, Lyne, Johnstone, Miller, Finn, Scruton, Moore, England, Kirmond, Ashurst, Sio.
Replacements: Walton, Arona, Simon, Molloy.
Salford: O'Brien, Carney, Sau, Jones, J. Griffin, Wood, Dobson, Kopczak, Tomkins, G. Griffin, Murdoch, Sarsfield, Flanagan.
Replacements: Evalds, A. Walne, J. Walne, Forster.
Referee: Joe Cobb (RFL)
Tom Johnstone's hat-trick helped Wakefield to a 32-18 win against Salford and gave them a third straight success under coach Chris Chester.