The 23-year-old Norway midfielder arrived at Parkhead in January after helping Stromsgodset to the title in his homeland for the first time since 1970.
Johansen broke the deadlock against Saints with a header a minute before the break before second-half goals from Leigh Griffiths and strike-partner Anthony Stokes sealed a comfortable win for the champions-elect.
Neil Lennon's side will wrap up their third successive championship with a win against Partick Thistle at Firhill on Wednesday night, if second-placed Aberdeen - who beat Kilmarnock 2-1 at Pittodrie - drop points at Ross County the previous night and Johansen acknowledged what would be an unusual title-winning feat.
"It would be two titles in about six months so it's going to be special for me," he said.
"Football is about winning. That's one thing you learn when you come to Celtic because winning is in the DNA and it's fantastic to be here.
"I want to win as many trophies as I can. It would be good to win it at home but I hope we win as soon as possible. It's good to just get it done."
Johansen joked that a recent haircut allowed him to see his header which gave Saints keeper Marian Kello no chance.
"I just needed to cut my hair so I could see the goal - so it was a good haircut," he said.
"It was a great ball from Leigh, I saw it and took the chance.
"It's important for midfielders to score. Hopefully you can expect more. It is not just for midfielders to control the game but to make goals and score them.
"I have only been here a few months but now I am starting to understand how others want the ball.
"I am enjoying my time here. I obviously had a good start and was playing a lot but you have to take a chance when you get it and keep working harder every day."
Griffiths, Stokes and defender Virgil van Dijk all had the ball in the net at various times only to see the flag up for infringements which led Celtic manager Neil Lennon to declare that the score could have been "anything".
"I am delighted with the performance, particularly, in the second-half, when I felt we were very good," said the Northern Irishman, who was quoted in a morning paper as saying he was surprised that his achievements at Parkhead had not led to an official approach by clubs in England.
"A wee bit wasteful in front of goals at times but I can't ask any more.
"It was another comprehensive win and another clean sheet. We are playing good attacking football and we are looking forward to every game."
- Sports & Recreation
- Leigh Griffiths
- Anthony Stokes
- Neil Lennon
- Partick Thistle