The then 16-year-old was making only his third top-team appearance for Rangers, having turned professional just three months earlier.
And, on one of the most dramatic days the old stadium has seen, SportTimes columnist DJ also ended a run of five League Cups on the trot for the Hoops.
He recalls being told he would feature in the Old Firm clash the day before the League Cup final and turning that night's dreams into reality the following day.
"It was just another Friday as far as I was concerned," said DJ.
"The game was on the Saturday and I trained with the first team and there were maybe 22 in the squad that trained the day before.
"After training I was called to the boot room with Jock Wallace and Willie Waddell and they said to me, here's half a dozen tickets, bring your family through to the game.
"I thanked them and they said, 'by the way you're playing. You'll need to get home early and get a good night's sleep to get prepared'.
"Of course I never slept a wink, thinking about scoring the winning goal time and time again and it was a dream come true the next day."
It was a significant goal for DJ, but also for a Rangers team desperate for silverware.
Johnstone told the Rangers website: "People often remember you for the first thing you do and because it was a cup final and a winning goal that's what I'm often remembered for so it was an important goal for me.
"At that time Rangers hadn't won a trophy for a few years and Celtic seemed to be winning everything so there was a bit of pressure on the management.
"They took a huge gamble having seen me scoring in the reserves."
He went on: "Jock Wallace and Willie Waddell just tried to calm me down because I was playing against Billy McNeill who was a legend at Celtic.
"They told me to get in about him and told me he'd try to intimidate me as a young lad and to make sure I stuck with it.
"I did that and afterwards the celebrations were fantastic because it was the first we'd won in a while so the fans were excited and so were we.
"I couldn't have any of the champagne though, because I was only a boy then!"