Hart has kept 17 clean sheets in the Premier League this season, more than his nearest challengers Petr Cech and Edwin Van der Sar.
And Mancini believes the 24-year-old could go on to become one of the country's best.
“I think Joe is young and can improve a lot but he can also become one of the top goalkeepers in England’s history if he wants,” said Mancini.
“If he thinks now he has arrived on the top and his work is finished, it would be a big mistake. He can improve a lot in his job because he is a fantastic keeper.
“He is as good as Cech and Van der Sar. He is younger than them but he can become like them.”
As well as topping the seasonal list for the most clean sheets in the Premier League, Hart now holds the City record for the most shut-outs in all competitions at 29, surpassing Nicky Weaver.