The powerful 24-year-old former Canberra Raider is the club's sixth new signing; following Michael Shenton, Jake Webster, Jamie Ellis, Jordan Tansey and Lee Gilmour in already having committed to Cas for 2013.
On the signing of Carney, head coach Ian Millward said:
"I'm hugely delighted with the capture of Justin. He'll perform a number of roles for us on the field as he is multi skilled.
"He will become a crowd pleaser with his robust running and dynamic style. No one will run the ball harder or faster than Justin in Super League in 2013."