The Hoops youngsters were 4-0 winners in the Glasgow Cup on Thursday.
The two sides met at Ibrox Thursday in a derby of the youth sides, as they contested the Glasgow Cup final.
Unlike the recent meeting between the senior sides in the Scottish Cup semi final, which finished 2-2 and saw Rangers progress on penalties, this was a comfortable win for Celtic.
The young Hoops ran riot in the game, running out 4-0 winners over the Gers to lift the competition for the third successive year, and Johnstone, who was one of the goalscorers, says it was brilliant for him and his teammates.
“It’s brilliant. I have never felt anything like that before,” he told celticfc.net.
Playing in front of a big crowd in a massive stadium and beating your rivals 4-0 – that’s what you want to do.”
“We had a game-plan and we knew how they were going to play. We stuck to our game-plan and I felt we did very well, defended superbly and we scored some good goals.”
The young Hoops can now reflect and celebrate their victory, which is made all the sweeter given it came against rivals Rangers.
Johnstone has however warned his team that they still have a lot to do this season as they look to secure the league title.
They have five games left in which to secure top spot and defend their title, in turn completing a domestic double and the teenager has urged his side not to forget that.
“We still have five games left and we can’t forget that," he added.
"We need to win the league and the job isn’t done. We are really happy and will celebrate winning the cup but we still have a lot to do.”