The Hoops midfielder began the demolition in the 28th minute when his shot was deflected into his own net by Perth midfielder Liam Craig and four minutes later he grabbed number two with another drive, before setting up Gary Hooper for a third before the break.
The Scotland midfielder scored from the spot 10 minutes into the second half to begin an argument of whether or not that was his hat-trick before substitute Charlie Mulgrew, on at the interval for Hooper, completed a night of misery for the hapless McDiarmid Park men when he made it five. Commons, who was given the match ball, said: "I will take it. Have I got it?"
He added: "I don't think the first one was going in for sure, it was more a cross than a shot. I'm sure the first one was on target, maybe not, I don't know. I was listening out to the announcer and I didn't hear my name so I thought they might have given it to someone else.
"It was a good move and if one of their defenders had not put it in, maybe one of our strikers would have got a two-yard tap-in. Certainly when I was taking the penalty I didn't think it was for the hat-trick, I was thinking it was for my second goal."
Neil Lennon believed Commons was right to claim a hat-trick in a "super performance" but was delighted with the way his whole side bounced back from the shock 2-0 home defeat by Kilmarnock in the Clydesdale Bank Premier League at the weekend.
The Hoops boss said: "I got everything I wanted. It was a fantastic response. It was a difficult couple of days for myself, scratching my head as to where that performance (against Killie) came from but we were back to our best and some of the football and the pace they played at was of a very high order.
"It was a big game, St Johnstone were going very well so to win the game so emphatically was testament to the quality of the players and their attitude. We moved the ball around quickly, we were aggressive in our pressing and tackling and that is the sort of team I want to see week-in, week-out.
St Johnstone boss Steve Lomas had no complaints about the result, saying: "Celtic were a class apart.
"They showed why they are champions of Scotland and were able to go to Barcelona and give them a game. You have to give them credit. It's all about bouncing back now for us."
Source: PA
Source: PA