The 19-year-old was called up for next Wednesday's Tannadice friendly against Hungary after making a major contribution to Kilmarnock's improvement in recent months.
Slater has made 16 starts since his full debut against Motherwell in November.
Johnston said: "He has been exceptional since he came into the team. The consistency he has shown for a young player has been really good.
"You can see he is improving as well and you can't ask for any more.
"You can see the quality he has got day in, day out in training. He was a star in the under-20s - week in, week out he was the best player.
"He deserved his chance and he has taken it. He has got to continue doing it because there is competition for places."
- Sports & Recreation