The Englishman led the Gers to the Scottish Championship title.
Warburton is up against League Cup-winning Ross County boss Jim McIntyre, Falkirk's Peter Houston and Jim McInally of Peterhead for the prize.
And while Johnstone insisted that winning the Scottish Championship and Challenge Cup was to be expected of the Gers boss, he suggested that their progress in the Scottish Cup - which included a victory over their Old Firm foes - is the factor that should tip the vote in Warburton's favour.
"For me, it is Mark Warburton who should get the prize. I know people will say that they expect me to choose Mark, but he has done a very good job at Ibrox this season," he told the Evening Times.
"It is always tricky when the awards are given out before the end of the campaign but Mark has done what he set out to do, and more, with Rangers.
"They were worthy winners of the Championship title and got over the line with a handful of games to go and he won the Petrofac Training Cup. He was expected to do both of those things, but few would have tipped Rangers to get to the Scottish Cup final and beat Celtic along the way."
While the Light Blues' position as favourites in both of the competitions they have won so far this season would mean that winning both would perhaps not see Warburton earn as much recognition, their Scottish Cup run, and the way that they have balanced the three competitions, has been hugely impressive.
The Hoops were their third Scottish Premiership scalp of the season, having previously knocked out Kilmarnock and Dundee, so it is safe to say that their pathway to the final has not been simple by any means.
But the way that Warburton has handled the expectation, and then surpassed it by enjoying further success in another competition, makes him the only candidate that Johnstone can envisage winning the award.