The Norway midfielder clinched the points with a 79th minute penalty after wide-man James Forrest had finished off a well-worked Hoops move from close range to open the scoring.
It took the Parkhead men eight points clear of Aberdeen at the top of the Scottish Premiership with both clubs having played 29 games albeit the Dons can reduce the deficit against Partick Thistle today (Saturday).
Ronny Deila’s side have only lost one league game this year and take four successive victories into the home match against Partick Thistle on Wednesday night which encouraged Johansen, 24, to say: “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.
“This was another important win and we just need to keep focusing now on every game over the next couple of weeks.
“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.”
Deila criticised the “terrible” pitches in Scottish football after the game, claiming the surface at St Mirren was another that was difficult to master.
The visitors were certainly not at their best but Johansen was glad the international break had not upset the Hoops’ rhythm too much.
The former Stromsgodset player said: “Some of the guys have had a few days off and some of the guys 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.”
Gary Teale is hoping his St Mirren side, who ended the game still four points behind Motherwell, are not cut adrift before the split, which comes after they play the Steelmen on Tuesday night and third-bottom Ross County the following Monday.
He said: “We need to make sure we’re within three points of Ross County or Motherwell after the next two games.
“If we can do that then that will give us a fighting chance.”
SCOTSMAN TABLET AND MOBILE APPS