The hard-working Norwegian midfielder was on target in Paisley after James Forrest had opened the scoring in the 63rd minute.
Johansen said: “It looks great for us now. It’s nine games left in the league and it’s going to be difficult to take us now.
“I don’t think we’re unbeatable, but you can say there is a huge confidence in the team now.
“I think we are starting to look fitter and we have much more of the ball because we are playing with a higher tempo and that’s the way the manager wants us to play.”
While Celtic found it hard to get their rhythm going in Paisley, Johansen feels St Mirren deserve credit for their disciplined performance.
But the 24-year-old was happy to get the three points following the international break.
Johansen said: “Some of the guys have had a few days off and some of the players were away with the international teams so it was good to be back together and be prepared for the rest of the season.
“St Mirren frustrated us for a while. They played well, especially in the first-half and credit to them for that.
“But we know that we had the ball so much that they would need to run. It was going to be difficult for them over 90 minutes so we knew it would open up.”