The 18-year-old forward, who has been at the club since the age of nine, penned a new deal in the aftermath of the 4-1 triumph at Celtic Park.
"It was literally straight after the final whistle," Johnston told the Celtic website.
"Once I got changed, I went straight in and got the deal signed."
Johnston and 18-year-old defender Anthony Ralston were praised by boss Brendan Rodgers for their performances as the champions swept Saints aside to extend their unbeaten domestic run since the start of the season to 43 games.
"I have played for this club for about 10 years and I have supported them my full life so to make my debut and then sign the contract and commit my future here is unbelievable," Johnston added.
"I have committed my future here for a reason. This is where I want to be and hopefully I can get many more appearances under my belt.
"Making my debut was one of my goals this season and to do that was a delight, and to sign the contract just topped it off.
"This has been a great season for Celtic, one of the best in the club's history, and it's something you want to be part of."
Celtic youngster Michael Johnston signed a new three-year contract after impressing on his debut against St Johnstone on Saturday.