Balotelli, a £24million 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."