Balotelli, a £24 million summer signing from Inter Milan, scored twice in the 3-0 victory against Salzburg in the Europa League, while Johnson was also on target.
The convincing victory means City have secured a place in the round of the last 32 with a game to spare.
On Balotelli and Johnson, Mancini said: "They could become among the best players in the world, strong players.
"They need to work hard all the time. They both scored fine goals but they must carry on."
Balotelli, who did not celebrate either goal, was given a standing ovation when he come off in the latter stages of the match and replaced by Emmanuel Adebayor.
Mancini explained: "That is just him. It is normal for him as he does it every day in training. He came off with a little knock.
"I think he can improve on that performance. He could run more and take more of his chances. He might have got two or three more goals."