Harley is so impressed with his teammate's much-publicised turnaround that he suggests Johnson one day might have an official leadership role within the team.
At the start of last year, Harley played a key role as the Cats gave Johnson an open-ended AFL suspension for another alcohol-fuelled misdemeanour.
It was a strong message that the enigmatic forward was on his last chance.
After five matches, Johnson was welcomed back into the fold.
By season's end he had earned All-Australian selection and claimed the Norm Smith Medal as the best player afield in the Cats' drought-breaking grand final win.
"To see him mature as a footballer has been fantastic, but to see him mature as a person over the last 18 months has been really rewarding," Harley said.
"He has as much impact as the leadership group – he's not in the leadership group, but I wouldn't be surprised in the coming years if he is.
"He's got that experience – if you want, the bad egg reformed – and he's certainly moved on from that, he's an exemplary citizen and a fantastic footballer."
Johnson said he had not made any dramatic changes from last year and just aims to keep becoming a better player.
"It's just continuous improvement, trying to improve my footy and trying to stay as professional as I can, just like any other player does," he said.
"There's probably been no difference, really."
Harley said the beauty of Johnson's play – or the brutality, if you are an opponent – is that there is always an element of surprise about what he does.
"There's a handful of guys where you have to let them display their natural talents and Johnno certainly does that, he's the igniter within the team," Harley said.
"But I don't think he lairises as much as a lot of people think he does, he's very much a team player and he's right up there on goal assists, things like that.
"You don't promote that sort of thing, it's more the instinct of the game – he's a special talent, no doubt about it and a great pick-up for us."
Pressure also obviously does not bother Johnson – if anything, like Hawthorn star Lance Franklin, it only seems to make him play better.
Combine that with his awesome talent and you have a finals performer.
"I'm not sure (it's) to the same degree as Buddy Franklin, but I do enjoy playing in front of a bigger crowd; it seems to motivate me that little bit extra," Johnson said.
"I'm sure the players who aren't playing in the finals – a couple of them said (at the All-Australian awards) that they were a little bit jealous that they have to watch us now."
