Celtic were unable to negotiate their way through Champions League qualifying, but they booked their place in the group stage of Europe's secondary competition thanks to a 6-1 aggregate success over Swedish outfit AIK.
Neil Lennon's side have already played eight matches in Europe this season, with Johnston featuring in six of those, and the Scotland under-21 international certainly feels his game is improving by playing on the continent.
"I think it will definitely help bring my own game on," the 20-year-old told the Glasgow Evening Times. "You want to play at the best level you can.
"If you play against better teams, it's going to help you improve. The opposition are better going forward, they will cause us more problems defensively."
Celtic have been drawn alongside Lazio, Rennes and the team that beat them in Champions League qualifying, Cluj, in their Europa League group and Johnston is confident they can make it through the knockout stages.
He added: "We want to beat the best teams put in front of us. Hopefully, we can get through the group into the knock-out stages. I don't see why not. Everyone feels the same way."