The Dunedin school continued its stunning run with a 29-26 win in Nelson this afternoon.
It means McGlashan will play at the prestigious national top four competition for the first time in school history.
Nelson fielded a much bigger team, and the Crusaders champion completely dominated the first 15 minutes.
But McGlashan gained confidence once it secured some ball and led 22-12 at halftime.
A stunning try to Max Hore made it 29-12.
Nelson scored two late tries and McGlashan showed a ton of ticker on defence to run the clock out.
Johnnies will play the winner of the Chiefs final at the top four in Palmerston North next weekend.