His production quadrupled over the previous year, finishing third in team scoring behind 20-year-old forwards Alan Lyszczarczyk and Cole Carter, was named the team's MVP as a result, and earned the Best Offensive Defenceman and Most Improved Player honours in the annual OHL Eastern Conference Coaches' Poll.
He was first on Mississauga with 31 powerplay points (5G, 26A), and had four points (1G, 3A) in seven games in a fourth-place finish for Canada at the World Under-18 Championship in April.
He's an elite skater, an excellent puck-mover and is one of the most talented players in the draft at transitioning the puck from defence to offence.
But he also knows there's a long way to go, despite the bulging comparisons to Ottawa's Thomas Chabot.
"My defensive consistency, for one," Harley said, rather bluntly, when asked what needed to improve to take the next step. "I think it was all over the place last year.
"But it's also what I've improved the most on, too.
"If you look at where I was at the start of the year, versus where I was at the end of the year, I think you see a big improvement. Just every facet of the game - gapping up in the neutral zone, D zone coverage, trying not to puck-watch with the three guys down low.
"I have some lapses defensively, but I've been working on that.
"And, I was shutting down the other teams' top lines most nights."