Hooker Johnson, 20, made his Betfred Super League debut for the Rhinos in their last match of the 2019 season after signing a four-year deal but then surprisingly retired from rugby league to focus on a career outside of the game.
However, this week, the former England Academy international has returned to the Rhinos - and immediately joined Championship club Knights on loan for the remainder of the season.
“Corey is a very talented hooker with good deception, running game and support play,” said York head coach James Ford.
“I enjoyed working with CJ at England and I’m confident he will have a positive impact on our performances. I’d like to thank everyone involved in helping this deal come through.”
Centre Hall, 18, made his Super League debut for Leeds last season and adds further depth to the Knights’ outside backs for his time with the club.
Both players are in line to debut against London Broncos on Sunday and Ford added: “Corey is a talented three-quarter who has the potential to make a significant impact on our performances.
“Corey is another player who represented England Academy and I’m excited to work with him again. Once again, I’d like to thank Leeds and everyone involved in this deal.”