The goalkeeper, who also spent time at Villa, made a late £10million switch to Wolves before starting at Nottingham Forest on Saturday.
Now wearing Wolves colours, the 31-year-old said his previous spell in the Black Country did not play on his mind when he made the move to Molineux.
Johnstone said: “Obviously the clubs are massive, massive rivals. But at the end of the day, it’s football so I didn’t think about that.
“This is a great club with great players, great staff, great fans.
“It is obviously difficult, probably, for the fans to see that, but for me, it’s a place I could come and play and that was what was important.
“I’m at a good age. I’ve got a lot of experience, I’ve played a lot of games and obviously I’m still young for a goalkeeper.