Johnson has consistently failed to live up to expectations following a £10million move from Manchester City last summer.
But the England winger is looking back to his best under Di Canio and smashed home a stunning goal in Sunday's victory at Newcastle.
Di Canio revealed: "I kicked Adam's bottom a few times during a session in a friendly way.
"I told him we can argue that it was the previous manager's fault, but now no more excuses.
"We have to be clear. It is all about the mentality. He was another guy that, when I watched a few of Sunderland's games, once he lost the ball it was a case of 'I don't care now, you try to work for me'.
"It was not because he is a bad professional, but now he has discovered that it is important to mutually work together. I told him if you work hard we will give you the opportunity to play your football in the right place. But you have to work hard physically during training all week.
"You hope that the words are enough. And, to be honest, he has done a fantastic job so far. We never had a bad word even when there was a session of running without the ball.
"Normally the fantasy player doesn't want to run without the ball but he was ready, so credit has to go to him.
"He took this as an opportunity to change and improve and there is much more to come from Adam."